we have 
to dispel harmful debuffs on allies. so why not expand on that? we could have a point and click ability/item to dispel buffs on enemies too. it doesnt have to be point and click, maybe its an aoe around a champ (like janna/kindred ult) or a targetable aoe (like soraka stuff). it could remove things like steroid buffs, shields, maybe even an enemy zhonyas! it would add another rock/paper/scissors strategic element to the game.
I could see a very interesting support champion having an ultimate that turns all buffs into Debuffs. For an example, lets say you've popped Ghost. You get ulted by this theoretical supoort. Suddenly, the 30% ms boost you have is now a 30% slow. Something like that sounds like it would be pretty interesting
I think it might end up in the same 'impossible to balance' category as Flash; it's just so overwhelmingly versatile and fixes so many problems, both offensively and defensively, that even if its stats were awful, it would still probably be worth picking up at least one or two.
Dispel was a shitty mechanic in even WoW. There's no way it would work here.
