Try not to go overboard with the minmaxing or give changes that don't make sense.

Pokemon: Rampardos
>New Hidden Ability: Simple
(The Pokedex entries always talk about how stupid it is, so it makes sense)
>Now learns Dragon Dance
(plenty of things in its egg group can learn it, with just as little justification)
>BONUS: Now learns Accelerock

Would he be OU?
Simple Dragon Dance gives him effectively a Swords Dance and Rock Polish in one turn, setting him up for a deadly sweep, but his frailty still means he's reliant on a Sash to actually survive it. Plus, there's still plenty of things that can outrun it after the speed boost.

Priority STAB is always nice for anyone, and it thematically fits with Rampardos's M.O.

Also, Simple Swords Dance on a Trick Room team (or even somewhere else, if you're comfortable relying on priority) is vicious.