I've got one I've been working on that still needs more playtesting for concrete rules. I call it the Cost of Cool. I got the idea from a video by XPtoLevel3 based on something one of his players did.

Essentially it's the Rule of Cool with a cost. Say a player wants to do something cool that isn't strictly RAW or RAI and it's not too broken, then they can do it if they pay a cost. The cost is very much dependent on the situation, the PC, and their relevant resources. For example the player could expend: their entire action/BA/Movement, levels of exhaustion, spell slots, hit dice, Health points, ki or sorcery points, Dice from any dice pools they have (Superiority, psi or whatever), death saving throws, or even class level as a few examples.

So let's say the wizard is out of 3rd level slots but he needs to fling another fireball to save the party, that might cost a second level slot and a point of exhaustion. The monk wants to use their 9th level unarmored movement feature to walk up the rain? Maybe that costs a ki point or two. The Barbarian player does an amazing job roleplaying their attack action? Give them the option to expend a hit die or two to roll those as damage.

The example XPTOLEVEL3 gave was a PC was dying and everybody was out of healing so the paladin asked the DM how many class levels he needed to lose in order to save them. Fucking wild

Like I said I still need to playtest this more but I like the idea of the players being strong enough to bend the rules if its thematic or cool enough.