FFXI has the monstrosity system which I found really fun, I'd love to see something like that in ffxiv.

Basically they player would pick a zone to go to and play as a monster, like a bunny or a lizard. You then levelled up your monster by killing other monsters, as you reached higher levels you would unlock the monsters abilities, skills and spells as well as bigger monsters in the family, like lizard > raptor > dragon. In certain areas other players could enter as a 'gladiator' and fight you.

For tanks the ability provoke, which was kind of like a spike aggro move, would lock the enemy target on you for a few seconds. I was thinking for FFXIV there could be something similar. Where moves with increased enmity on them would instead force the opponent to target you, not lock them, just make them target you, this would make the tank a nuisance that they'd want to kill to stop the constant messing with them. While provoke could be used to lock the target on for a few seconds just to make it slightly more useful.