The cops, They have been around since the start and can not be stopped. In V they have become stronger and in the next GTA they will be Super Sayins!
But seriously it would be cool if the main character was the antagonist. They would create their own problems and problems for the people they work for. Rather than someone betraying them, they would betray others for a upper hand. Not sure how it would work but would be interesting to see.
Good idea. Although Devin and Steve were the shown antagonists of V, Michael seemed to me more antagonistic, well after all, he's a three bit, retired criminal who is a back-stabbing bastard.
I think "a good antagonist - bad protagonist" idea, like Niko Vercetti suggested above, would be something fresh and unexplored for GTA. I dunno, maybe a law enforcer like Vincent Hanna from Heat, a character who puts his all efforts to catch the protagonist who is a treacherous, disrespectful bad guy. Or maybe, law enforcer like Slocumb from Thelma & Louise, who actually wants to help the protagonist shape his life but the protagonist is too dick to see it or accept this guy's help.
I'd like to see a brutal, fierce and sociopath antagonist, yet he should have the good state of mentality which a good antagonist should have. Think about a drug lord who beats the sh*t out of his men when they judge his style or performs brutal tortures on his foes. Besides using his violent side to impose authority, he should have a "wise, opportunist businessman" side when doing business. Although, Diaz was actually a short-tempered, presumably violent drug lord, I think we couldn't see much of him in VC. So I think this style of antagonists should be explored more.
Also, I really enjoyed Billy Grey from The Lost and Damned, I'd like to see an antagonist like that again. Billy was a real strong and capable man, IMO; he was brutal, short-tempered, violent yet he had big lobes of brain: he was good at manipulating people (look how he managed to make Johnny attack the AoD Clubhouse in order to sit on a payload of heroin during Action/Reaction, and he managed to turn lots of people against Johnny and his faction during The Lost civil war) and making opportunist, yet "equal" deals (his deal with Triads during Chinese Takeout from TBOGT, leading to the events in This Sh*t's Cursed). A kind of this sneaky yet clever bastard could work well in another GTA game, especially as a part of small gang.
I'd have lots of ideas and opinions when it comes to antagonists that are pretty important parts of a fict. However, I think the antagonist types I've mentioned above would be my favourite choices.