This one isn’t a single mechanic, per se, but:
I’ve always wanted to play an action game where you heal people but don’t hurt them… but like a hard game. Heart Souls, as it were. You as the MC have amazing healing, augmentative, and shielding abilities, but the healing and augmentation is only for others. You yourself have only mobility and shielding. You have to wade into a series of massacres and turn the tide by surviving long enough to help the victims repel the aggressors. The more trust you earn from those you are helping, the more you can influence them to take actions that facilitate this, meaning most situations start with you mitigating trauma, but hopefully end with you directing the resistance somewhat due to rallying the victims.
Ok that’s more a game idea than a mechanic.
As for a more singular mechanic, I would like to see a RTS or SRPG game that doesn’t use a turn counter or action cooldown in the traditional sense, but instead lets any unit attempt to act whenever they want to, but bases part of the success chance (and other variables) upon how soon they try to act. Imagine a normal-looking universal cooldown timer, but as it fills, each increment adds a certain percentage of success potential, while failure would have different effects based on severity. 100% filled would mean full potential and 0% negative results for a failure (beyond, well, not succeeding.) 1% filled would mean the least potential for success and the most harmful effects for failing.
As all your units have their action meters (or whatever) building, you are constantly weighing the advantages of acting sooner vs acting dependably. Gambling initiative vs competency, as it were. Stats could just affect this % chance based upon what actions are taken that they govern. For example, having great ranged accuracy but bad mobility means taking a bow shot with a half-filled timer would have a much better chance of succeeding than trying to dash with a like-filled timer.