We know that freeplay allows for switching between the three guys on the fly, so I'm pretty sure that rules out a surprise death of one or more of them.
I'm willing to bet that at some point, Trevor's trailer gets blown up. So for his part of the ending, he uses some of his heist money to buy a brand new trailer and possibly a new meth lab (I'm assuming he has one already, which is what will cause the explosion in the first place).
Franklin's dog has puppies, and the other guy finds out his daughter or wife (or both) is pregnant with a litter of her own.
In a world built on hope and faith, I am the realistic idea. The dream come crashing down.
Micheal's son gets kidnapped by some gangsters from his past. He tells the boys they want a lot of money to give his son back, and that he's in no shape to be a part of it, other than help plan it. He tells them about this shipment of cars coming in at the docks, and that if they can nab it, they can trade for his sons life. Trevor and Franklin drive down to the docks only to find out that there's no commercial dock in this area. They head back to Micheal's house only to find his son is already there, and Micheal is gone. Turns out the only way Micheal's son was going to live is if Mike gave himself over to the mob without a fight, and he knew Trevor and Frank wouldn't just let him do it. You then play as either Franklin who races along the highway looking for Mike, or as Trevor who flies a chopper in the opposite direction, doing the same. Neither find Mike, but the following day his body washes up on a beach, his head caved in, and tongue missing. At the funeral, Mike's therapist hands Franklin and Trevor an envelope each.
Franklins tells him that while money isn't easy to come by in this life, it isn't the most important thing. And that he should find someone who loves him for what he is, not what he has. And until he loves himself for who he is, he's never gonna be able to do that. Franklin smiles, folds the letter up. He turns to Trevor and Trevor is doing the same. Trevor asks what his letter said, and Frank tells him "Motivational speech. But I think the kind I needed to hear. What about yours?"
Trevor smiles, and looks over his shoulder toward the grave. Muttering "the same."
Credits roll. Music plays. After a good minute of unskippable credits, Micheal's voice is heard. It tells Trevor that Mike knew what was going to happen, and went to it knowing that it was what he had coming. He then tells Trevor that no matter what he thinks, there is no way to make this right, or better, and that revenge will only bring the same amount of trouble onto him, and Franklin. And to just let it go.
The credits then stop rolling, and we come into view of the head of whatever family that had Mike killed. He sits in a large comfortable chair, smoking a cigar, and all of a sudden, he hears a noise. Looking toward the window, he then comes face to face with Trevor, sitting inside an Apache Helicopter. His mouth dropps open, and we hear Trevors voice. "Sorry Mike. But no." Bullets rip into the head family guy, his henchmen, the building, and finally the camera itself. Bringing us to the real end credits.
This would probably mean that free play would consist of Franklin and Trevor, with Trevor getting access to that Chopper and Franklin just getting a bunch of money to buy whatever.
GTA endings have never been great, they always leave with the character saying they are out or finally finished with whatever they have been doing crime wise. I dont expect it to be much different I would love a dramatic sensitive ending like RDR or MGS.
Honestly as long as they end it in a way that the characters seem like they will and want to keep on living the life as a criminal I will be happy because nothing is worse then the characters saying they will never kill again or steal again then later you can do that just after the credits roll.