High Noon 
Premise: The speed stat no longer exists. In fact, the turn order no longer exists ! All actions happen simultaneously, and the results are evaluated at the end of the turn.
So let's say you have Cinderace vs Metagross for example. Cinderace clicks Pyro Ball, killing Metagross. In a normal game, because Cinderace is faster, it stops here. But here, because everything is simultaneous, Metagross also gets to click EQ and kill Cinderace back.
Another thing to consider is the way healing works here: Since you can heal even on a turn where you would die, until a turn ends and everything is properly evaluated, you can overheal (but all overheal returns to 100% at the end of a turn), or get overkilled, because else things get messy (for example if you can only be damaged down to 0% then spamming recover effectively makes you immortal). To avoid ties, if the last 2 mons kill eachother, the one who got overkilled the most loses. So in the case of the Cinderace vs Metagross scenario from earlier, assuming they were both at 100% and each of them was the last mon on their trainer's team:
(They would run max HP/bulk cause speed is useless.)
252+ Atk Cinderace Pyro Ball vs. 252 HP / 0 Def Metagross: 318-374 (87.3 - 102.7%) -- 18.8% chance to OHKO
252+ Atk Metagross Earthquake vs. 252 HP / 0 Def Cinderace: 312-368 (85.7 - 101%) -- 12.5% chance to OHKO
Ok well I didnt expect them to live actually but lets pretend they both got the absolute maxroll -> Cinderace wins, because he did the most damage.
Status moves that would affect the other pokemon's move (Taunt, boosting your defenses, setting up terrain or a weather condition, etc...) are considered to take effect starting from the next turn (although this could be changed).
An exception is priority moves, which will still behave normally; if you kill something with priority, everything will happen as it would in normal game. Same if you die before using a negative prio move.
This has all sorts of interesting implications, such as:
- Bulkier mons or mons that have huge offenses but no speed become better.
- Purely offensive set up moves like Swords Dance or Nasty Plot aren't as good anymore, as even if you can ohko everything, you'll still take damage from them before. Stuff like bulk up/curse is favored instead (defense boosts only apply starting from the next turn however, consistently with the way status works).
- Sturdy becomes much better, as a mon living on 1% guarantees it will get another move off, even if it's slow (Any Forretress fans in the audience?)
- All pivoting is slow pivoting. This makes defensive pivots much better, but it's also weird because it's not like your switch in can outspeed on the next turn.
- Drain moves become much stronger, as they allow you to stay alive on a turn you would have otherwise died (Primarina, hatterene, and probably some grass types with stab giga drain become much better)
- And probably a bunch of other stuff, too ! This premise seems decently unique, I don't think I've heard of something like that before. There are probably still some rule points that need clarification I guess, but thats why the workshop thread exists :)
(I called it High Noon because in cowboy movies everyone turns around and shoots at the same time in the duel scenes, kinda)