More flags more fun
What if every Pokemon could put up a fight even till its dying breath? Well in Last Will your Pokemon will and will be able to pull off some last ditch moves which could ultimately changed the tide of the match.
Mechanic: Every Pokemon will use the move in their last moveslot before fainting in battle only if they can normally perform the move (such as not being able to use Stealth Rock under the influence of Taunt.)
Clauses: Standard OU
Super Move Clause - You may only have two Pokemon with the moves: Blast Burn, Hydro Cannon, Frenzy Plant, Hyper Beam, Giga Impact, V-Create, Explosion
Bans: OU Banlist
Endeavor - always gets a kill unless you are a Ghost-type
The main strategy will be able to take advantage of moves that have too much drawback in a regular match but have no serious drawbacks now. Some examples would be using a move that has a recharge time after its use like Hydro Cannon or Blast Burn. All of the suicidal moves are great options to like Explosion, Healing Wish, and Memento all give the user a turn of momentum and without the cost of sacrificing a Pokemon just for the move. Finally there are a lot of supportive options that could be used within the last slot as well like hazards so the opponent can't Defog them the turn they are set up or the opposite, remove any hazards last second so the next Pokemon can come in safely.
Q: What if a Pokemon faints because of passive damage (Burn, Stealth Rock, etc.)?
A: That Pokemon will still use its "Last Will" regardless of how it faints.
Q: How will Destiny Bond / Grudge work if used as the "Last Will?"
A: (Hopefully) These moves will not have their normal properties and it simply fail. If it is unable to be coded that way they will be banned.
Q: Can I remove hazards with my Last Will?
A: Yes but only through Defog, Rapid Spin will not work.
Q: Do Choice Items prevent me from using my Last Will?
A: Yes unless you are locked into the fourth move itself. Even though the Pokemon could technically use the move if it was itemless, it is stopped by Choice Band, etc. "Encore" like effect.