Looking at the cleric as a bundle of resources for a moment:
Both wells and clerics generate water. A well accesses underground aquifers* and can generate larger and smaller volumes of water depending on local circumstances. Furthermore, most liquid intended for human consumption is vaguely alcoholic as a purifying measure.
A human will consume 3-4 liters of potable water per day and some more in other activities. A cleric, therefore, can't make all that much water relative to a village's needs in a day where 0th level spells are restricted. Therefore, from a practical matter, it's much easier for the cleric to cast "purify food and drink" on the well or on a locally drawn reservoir than to create water.
However, this is not an interesting case. The interesting case comes from economies of scale and opportunity costs.
Treating a cleric as a well means that for every "well-like interaction" the cleric cannot be doing other things. Therefore, if a cleric did serve as the village's water source (cleric-as-well) they would start the day by filling a container with water and then go about their normal business.
Therefore, if you want a poisoned well scenario here, you must poison the cleric.
From a more practical matter, this one interaction costs a minute or two of time and doesn't place much stress on the cleric. It does, however, increase the villages' dependence on the cleric and reduces the overall food supply of the village (see purify food and drink, which is an "I can't believe this wasn't stored in a fridge" spell that does enormous amounts to increase food efficiency by preventing spoilage. Thus, it's the cleric's best interest to see that the well is purified since more people will benefit.
There is also a single point of failure here, given that the cleric quite literally has the capability to deny life to anyone in the village by simply choosing her spell choices differently. The villagers may not like to be so dependent on the caster.
Creating an economy in a 3.5 world is interesting, depending on the social constructions you have available. I recommend reading the various tomes here by K as to why Feudalism. Considering that the new world expedition was funded just to have easier access to pepper, entire plots can hinge around finding likely students who can learn to cast purify food and drink. Poisoning the well could be literal (causing starvation) or figurative (magical well of energy that powers divine links, etc...) or political (a new cleric's in town, and she says that casting these spells defiles the miracles granted by the gods.
Given that this is not a post-scarcity society, resource deprivation is always possible. It's just may attack different links in the network.
For an interesting thought experiment, consider the destabilizing potential over a hundred years of a wondrous item of fabricate, a wall of fire made permanent, and a wall of iron spell.
Edit:
@Cross notes:
"The difference is that in Pathfinder Cantrips and Orisons are effectively "At Will". This means you can create water till the cows come home."
Then you get into the relative utility of time and economies of scale. Every round spent producing water is around that isn't being used to, for example, purify food and drink. (I hope I made that point clearly enough in my answer) In the grand scheme of things, one cleric preserving a village's food supplies across winter means a whole lot more accessible food from their environment. Probably far more than the consequences of refrigeration. Farming patterns shift to high-calorie crops instead of preservable crops, and villagers can start achieving remarkable efficiencies.
For one thing, the average calorie intake of each villager goes up tremendously. Which produces healthier villagers who are more able to meet their adventuring capacities, which produces more clerics. This sounds like a virtuous cycle that will quickly strip the lands of reasons for Feudalism, which means that some counter-force is required to remain "expected authenticity." Don't even get me started on the consequences of rings of sustenance on a village's production capacity.
* Aquifers do not generate happy thoughts in dwarves.
In Mystic Empyrean the players create the world as they explore it with a mix of individual authority, shared authority, and random card draws. It's non-traditional in a lot of ways though, so not everyone's cup of tea. It is definitely a worthwhile example of how such a system could be built. Studying the interplay between the system mechanics, character mechanics, world-generation mechanics, and setting conceits to determine how it ticks could be enlightening.
Key to the on-the-fly world generation are the game's authority structure and the nature of the setting and characters.
Authority is shared by all players:
- Everyone has a character and GM duties rotate each encounter round.
- Everyone has opportunities to declare or randomly generate what the group encounters next.
- Important facets of the world and game are divided up and "owned" by different players so that players can appeal to an authority and so players can contribute in ways they enjoy. Different groups will divide things differently, so some might share creating all new realms and NPCs, or one player might enjoy making NPCs and "own" NPC creation, or one player may own a story arc, or each new realm will be created and owned by a different player, or some other division of ownership.
The nature of the setting is specific but also not particular:
- The characters are mutable immortals whose personality determines their nature, appearance, and powers. Personality is challenged and changed by play, influencing nature/appearance/power.
- The world is multi-faceted, with many different realms with vastly different physics and realities all in one shared "multiverse" that the PCs can cross between.
- All realms but the starting realm are lost within all-consuming mists where there is no existence or time, due to an ancient catastrophe. Gameplay is about restoring lost realms into the fabric of reality and exploring/exploiting/helping/ruling/whatever those realms.
- The setting defines seven elements that every thing and every action is composed of and aligned with, and can use these in a card-draw random generator to determine everything from a realm's inhabitants' government style to the fantastic geography to the realm's possible technologies.
- The player characters are rebuilding the world in their own image as they rediscover lost realms and influence how they re-integrate with reality.
The world-generation system, then, relies on the fact that authority is shared and on the nature of the setting to harness the group's creativity to build out from the starting realm. The shared authority means that small contributions build up in unexpected ways into interesting, engaging places and events that nobody needed to construct (or even could have predicted) beforehand. The nature of the setting means that there is lots of room to build anything the players can imagine (really, anything is compatible, the way the world is defined in the book) and individual players can lean on their creative strengths.
The setting also means that there are natural bounds to play, so the mode of play switches to a creative building mode only when one of those limits is deliberately crossed by the group in order to discover what's over there. It handles during-play world creation as well as between-play solo creation, depending on how ownership is apportioned for the to-be-discovered piece of the world. The conceit that the PCs are shaping the rebuilt world in their own image makes the creative play mode parallel what is happening within the game: the player creating a realm represents their PC rediscovering a lost realm and influencing it's unfolding back into reality with their own personality.
The default way of playing is very non-traditional, but it also supports a more traditional GM/players division of labour simply by giving one player ownership of more kinds of things in the world – if all new realms are owned by the "world player", then one player can craft the world to their vision while the "character players" manipulate and explore it. It also spends a few paragraphs on using the system for different genres.
It's not a generic on-the-fly world-building system by far, but it's an interesting game technology and the only one that I know of that actually works seamlessly during play to give both structured results while being flexible and who and how it's used.
Best Answer
Dungeons & Dragons 3.5 is not well equipped to describe the world prior to the Event. If you try to do so, you’ll be fighting the system every step of the way: 3.5 is extremely high magic, and expectations of high magic are baked into the system at every turn (most especially with how CR is determined, and by extension published modules). It certainly works better at low levels, but even at low levels, having some people play barbarians and crusaders and others play, well, commoners is not going to work well within the system. Anyone who wants to play a magical class is going to have to slog through a few levels of being useless dead-weight, and the other characters are going to need a very good reason for carrying that around.
Also, the claim that low-level spellcasters spend significant amount of time with slings and crossbows is not, in my experience, particularly accurate. Low-level spellcasters must use spells judiciously, which means, usually, they try to cast just one spell to give their side some major advantage (usually entangle or grease, sometimes sleep or color spray; other options are possible), and then allowing the mundane classes to take advantage of it. They might take potshots with a crossbow or sling, but that’s mostly just for something to do: they’ve already done their major contribution to the battle.
What you propose eliminates that contribution to the battle.
Finally, the world of Ishtar pre-Event seems to be lacking much in the way of major conflict. The players might be sent on your typical rat-killing quest, I suppose, but mostly this is not a world where the rules of Dungeons & Dragons are appropriate: those rules are really designed for dungeon-delving and dragon-slaying. They have little to offer a world of prosperity and comfort.
My solution would be to run the game in a different system entirely, prior to the Event. Something rules-light and narrativist; these seem more appropriate to the tone of pre-Event Ishtar, and the issues of magic need not even come into play. Where Dungeons & Dragons is ill-suited to this, Fate would be excellent. In fact, considering the relatively-brief time spent pre-Event, I would probably go with Fate Accelerated Edition.
I would have the players still pick their post-Event D&D class and ability scores while building their pre-Event Fate characters. I wouldn’t have hard-and-fast rules for what each class and ability score array means in Fate, but I would look for some commonality: someone with high Intelligence after the Event would presumably have had strong skills with Clever approaches prior to the Event.
Then the Event happens, and they become their 3rd-level D&D-character versions of themselves, complete with magic as appropriate.
I can personally vouch for Fate being used this way; I played in a giant-mecha-style game, where the pilots were all Fate characters and the mechas were all D&D characters, and the mechas were supposed to enhance the natural abilities of the pilots, so this kind of commonality was required. It worked very well. For that matter, I’ve used Fate-inspired subsystems to replace D&D’s skill system wholesale, and that also worked well.
One difficulty here is the transition period. Spontaneous spellcasters have it relatively easy, as do divine spellcasters in general, but wizards (and wu jen) have an issue with regards to the spellbook. I would handle this through allowing them, at first, to use their spell slots on read magic. This spell is one “all wizards” can cast, and though that is usually something they simply have studied well enough to memorize without a spellbook, it could be that the Event itself imbued them with this ability.
I would then have the world itself provide the spells. Considering all of the physical and meteorological changes in the planet, which is likely caused by this new magic, I would also have arcane patterns appear in nature: rock formations, vines, even the rapids of a river. Temporary events, too, could work: the way a particular bush burns, the way tea leaves land in the bottom of a cup, and so on. Seeing magic in the world around them, and understanding it with their new read magic ability, allows these wizards the opportunity to begin to craft spellbooks. It also makes for a really fun way for you, as a DM, to offer special “loot” in the form of spells they can copy.
Since buying spellbooks already filled will be impossible, and they won’t be starting with a spellbook with months’ or years’ worth of spell-scribing going into them, I would also consider reducing the time required to scribe spells (but not other costs). Then again, wizards are so powerful that I might not, or maybe only for the lowest-level spells (cantrips, 1st- and 2nd-level maybe).