And this is why Rockstar never makes direct sequels (meaning continuing where the last game stopped) because after the antagonist is killed, there is little to no more reason to continue a protagonist's story. And that is why, my children, we never get to play as the same protagonist for more than one game.
Unless they can introduce a new Antagonist who fits into the storyline.
The seeds are there if they want to build off of them. During the Casing the Jewel Store mission as Michael and Lester are going to the car and discussing who they could use for the Crew, Lester mentions he's been "working with someone, but he's too Unstable". Maybe Lester's other partner is a combination of Michael and Trevor, gets angry/jealous about not being brought in on the Big Score and goes after Michael, Trevor and Franklin in a Grand Conspiracy.
I think that's supposed to be a reference to the GTA Online protagonist.
Also it's mentioned Solomon Richards and his Son have a bad relationship. Maybe the Son bumps off Solomon and Black Balls Michael from Vinewood.
I'm pretty sure Ira Richards only purpose in the story was to give a lead to the police in the Leonara Johnson case. Soloman calls him a moron for telling the police that his father may have been involved in her murder.
Martin Madrazo is still around, with the calls and emails Patricia sends to Trevor there's fodder to reignite a feud there.
I doubt it, considering Madrazo seems to be absolutely terrified of Trevor and Michael, and when he had the chance to kill them both he kept his distance.
Simeon Yetarian could be revealed to have ties to the Armenian Mafia and could come after Franklin. The Lost MC could declare War on Trevor.
You can kill him, so I don't think Rockstar would do that. The Lost MC was entirely wiped out of Blaine County and no other chapters seem to take notice, anyway.
Don Percival could decide the crew owes him a few favors after the trouble they caused for Merryweather.
This, I agree with. Considering he was behind Merryweather throughout the entire game and we never even saw him in person, I think this is the most likely thing to happen, especially because at the end of his emails to Michael and Trevor post-game, he warns them to "never f*ck with me again". Lots of potential there.
Combined with Michael’s F'd up family life, Trevor's Psychotic behavior and Franklin's ties to professional idiot Lamar Davis there are a lot of ways for them to do a continuation of the story.
Maybe something like....