I've been running a Pathfinder 1e game for several years now, for a group of very inventive players that come from all sorts of walks of life. This situation is from the game session that happened over the weekend.
One of my players set up to create a (honestly, rather amazing) special forge for their character.
This character is a 17th level Gestalt Sorcerer/Witch, with a heavy arcane blacksmith theme to it. She recently gained the favor of the patron of her family (a mighty red dragon), and was rewarded with an entire volcano for her services.
She decided to turn said volcano into a place to create magic items, as some sort of dedicated, magical forge. Her plan is for the place to provide several assistants made of lava, that would copy the spellcaster movements, spells, and enchantments.
Those assistants would be able to forge, smith, and work with metal and magic with the same proficiency the user of the forge would. They wouldn't improve one's abilities to create anything – instead, they would multiply the output of said spellcaster, provided they have enough materials to make the entire batch. So, for example, if one mage would create a set of five [Flaming Longswords +1], they would still need to provide the needed materials and costs to make the five individual swords – but they would be able to finish it in the same time they would take to make a single one. The forge would save time, not costs.
The forge itself would provide the magical energy to fuel the several copies of the spells that are needed to create several magical items at once by tapping into the leylines of the planet. This process is already defined in the setting's fiction as possible, as one of the races actually uses this as a form of sustenance for their addiction to magic.
The rules for creating magic items are, despite their shortcomings, there. They exist, and as such they can be used as guidelines whenever the DM needs an idea or two to create custom magic items.
That said, I've found nothing regarding creating magical places. Those exist in the fiction in abundance – an island enchanted to change places from time to time, a mage's tower with several magical effects to boost its owner magical abilities, a magical city-tree that grows forever and grants the people that planted it immortality, floating magical cities, and so on. Those are usually made – again, in fiction – by regular spellcasters with enough time, material, and power to do so. Those aren't epic mages with three dozens of levels – they are people on their 15ths and 17th levels, far below godly status but still with a good share of power in their hands.
Sure, one could always create a magical place by treating it as a collection of several magical items, but this doesn't make justice to the uniqueness those places usually have, nor the magnitude of the effects they usually hold. Thus, it is very hard to eyeball what would be needed to to make such an endeavor possible.
My problem:
Is there a set of rules that could be used for guidelines to create magical places?
I would prefer Pathfinder 1e material, but D&D 3.X/4e/5e is also acceptable. I would prefer to avoid untested homebrew, but if you have something you tried on your table and it worked, that would also be acceptable.
Best Answer
For your real question: just give it to her.
You already like the concept, you already want to do that, you already have the mechanical and story parts, and you already know there isn’t any magical-places system to fit this in. There is no need to create or discover such a system, then use it to back into an answer you already have.
For your general speculation on why Pathfinder and D&D 3.x don’t have unbounded, roll-your-own-artifact effects available at relatively low levels: because it isn’t that kind of game.
D&D and derivatives are games that fundamentally grew out of adventures where you kick down dungeon doors, murder or sneak past everything inside, and stuff loot in giant sacks. As a system, they handle small-unit/SWAT tactics well, and everything else poorly. I have a lot of fun in the space it handles poorly (necromantic industrial revolutions? Yes please!), but they break really fast.
I am not convinced that it is possible to have a balanced game that both (1) uses rules, and (2) allows for powerful, arbitrary magical effects. Probably the closest system I’ve seen to what you are looking for is Exalted 2E’s manses, but Exalted simply does not care about balance: the core game assumes you are playing one of the most personally puissant entities in existence, so it doesn’t even matter if your house makes you immortal.