My question is similar to this question about a house-rule that the initiator of combat acts first but skips their first turn in initiative, but about the opposite situation. The author of that question assumes the enemies are aware of the party; in those cases, I prefer to use initiative as normal.
I am focused on the opposite situation: the party has managed to ambush the entire group of enemies. If enemies did not notice the party, the party has time to set-up and choose whoever should start combat.
Very often my party comes up with interesting traps and ways to initiate combat. The problem is that in 5th edition, once the combat is initiated, everyone rolls initiative and the combat goes from there. Specifically, my issue happens when an entire group surprises the other.
Here's an example:
The party is all hidden in the shadows. 3 ogres lurk in the campfire in front, and have not noticed the PCs. The plan is simple. Wizard opens the combat with Faerie Fire, and all the martials will have advantage on their attacks. A single round, at most two, should be enough to wipe these Ogres.
What actually happens depends on initiative: everyone rolls initiative, Wizard is last. Ogres are surprised. Rogue and Fighter either move in to attack without advantage (and are possibly hit by Faerie Fire), or ready a single attack for after the Wizard has cast his spell. Ogres do nothing, since surprised. Wizard is last, and when Faerie Fire is actually cast, the damage output of this surprise attack is sub-par. If Wizard came first on initiative, the whole plan would be perfect.
The worst part of this is that there isn't even a justification for this happening this way. On a stand-off, initiative represents reflexes, and it works well. But here, it doesn't seem to make sense that, if all players are waiting for the Wizard to do something, that they will be hindered (no multi-attack, no off-hand attack, etc) if, for some reason, the Wizard rolls low. Narratively, they could just go "hang on, seems like the Wizard is distracted, lets remain hidden, and re-roll initiative". Initiative is an abstract concept, and during the actual combat, I'm fine with how it works, just not for this combat initiation against a surprised group.
My House Rule: When a group attacks a surprised group of enemies, everyone rolls initiative as normal. For the first round only, the attacking group selects an initiator to act before the highest initiative attacker, initiating combat. This initiator will not act in its actual initiative position on the first round, but will do so on following rounds.
Example: Ogres have initiative 25, 20 and 15. Wizard has 5, Rogue has 17, Fighter has 22. With this rule, Ogre acts first, does nothing due to surprise, stops being surprised. Wizards is next, acting as combat initiator for the party, playing before the fighter in this round. Fighter is next, then Ogre (surprised), Rogue, and Ogre (surprised). In the following round, the order is Ogre, Fighter, Ogre, Rogue, Ogre, and finally the Wizard.
It is a simple change. The group initiating combat, because they have the upper edge and time to prepare, chooses a person to go first in the first round. After that, everything else is exactly as before. If it happened that this person actually had the highest initiative check, nothing would be different from normal.
Thoughts on this rule? Would it imbalance things, or can it be taken advantage of somehow? I think it is a decent way to join narration, planning, and mechanic execution. It helps those parties relying on Assassin Rogues or on AoE initiators.
Best Answer
Tactical parties can coordinate better
This is obviously the intent of your change, but it bears stating explicitly; this will tend to advantage parties that behave tactically and rely on support, buffs, and/or control. In my experience, rules that advantage tactical behavior tend to help the PCs more than their enemies. In this case, the PCs may take advantage of this rule almost every time they generate surprise, while NPCs probably won't benefit as often, unless you regularly run encounters against ambushing bandits with support casters. I doubt this will be a huge problem, but it is something to consider.
Poorly coordinating parties may bicker
The bigger issue, in my view, is that your rule appears to require consensus between an entire side in designating their initiator. If your players aren't super strategic, or if there are multiple characters who could benefit by going first, or if some of your players just like to show off, they may not be able to agree easily. In-group bickering can be a problem in a lot of situations already, but this adds another potential sticking point.
Assassin rogues will benefit tremendously
The assassin rogue's third-level feature grants critical hits against surprised enemies. This feature can be quite powerful, but typically requires the rogue both to successfully sneak up on or otherwise surprise their opponent and beat their initiative role. Under the default rules, the risk of rolling low initiative can balance the possibility of an auto-crit, and also strongly incentives the rogue to choose features that improve their initiative. Under your proposed house rule, the rogue could request to be their side's initiator every time they roll a poor initiative, rendering this feature much more powerful and reliable. This change obviously only matters if you have an assassin rogue in the party, but if you do it could be a fairly significant power boost.
Existing rules and variants may already accomplish what you want.
As a general rule, I like to introduce house rules only when the normal rules really aren't working. In this case, you're adding a small but potentially significant bit of complexity and decision-making for a fairly niche situation. Moreover, there are existing rules which may adequately solve your perceived problem. First, as you've noted, players can simply ready their actions in order to allow one PC to act first. The ready action appears intended to model the exact situation you describe, although as you note there are some mechanical downsides to using it. Additionally, there is the "Side Initiative" variant found on page 270 of the DMG, which allows entire sides to act at once, with members of a side acting in whatever order they choose. This variant has its own advantages and disadvantages which are highlighted in the DMG and discussed in this question.