According to GTAWikia, Michael was born on the East Coast. That could be Vice City or Liberty City. However, I couldn't find anymore proof so I am going to skip that idea.
The game takes place about 10 years before GTA V. Michael and Trevor are successful bank robbers in San Fierro. (In Trailer #1, Micheal said he moved to Los Santos. He might have not even lived in San Andreas at all but that will complicate this idea and I might post later about a fix to the story.) The game will have the same synopsis of GTA V: You do a bunch of mini-heists that ultimately lead up to a major heists with big payoffs. (Franklin won't be in it because 1: He doesn't fit the story and 2: He'd be like 15. However, an up and coming bank robber named Marcus will help them in the getaway) You can switch between Micheal, Trevor, and Marcus at nearly any time similar to GTA V. Franklin, however, will make a cameo. Possibly as a witness to a heist like Luis Lopez in Three Leaf Clover.
Now this is where I discuss the ending.
ENDING #1: After the game's final heist, Micheal and Trevor get busted. The FIB make a deal with Micheal and give him a mansion in Rockford Hills, Los Santos. He must retire from his life of crime if he accepts. If he refuses, he dies. He tells them only if Trevor is spared from jail. The FIB agree but don't tell Micheal their plan. You see, Trevor bought houses, drugs, and cars with his money. (Note: at this time, Trevor isn't as addicted to drugs as he is in GTA V) The FIB seizes all his assests and sticks him with a trailer in Blaine County. This leads to the rivalry between Micheal and Trevor. This ending is good story-wise as it describes the life of the two prior to GTA V. You can still play as both of them after this ending, however, if they see each other, a fight might initiate. Marcus was a minor accomplice so the FIB forget him, and he keeps all his belongings.
ENDING #2: This ending is nearly identical to ending #1. But instead of instantly getting busted by the FIB, Micheal and Trevor say before the heist that this will be the final one. After it's over, Micheal and Trevor bask in the glory of their victory. This ending is good for the consumer because it allows you to continue to play as the two as friends. Shortly after this ending, the ending to #1 occurs (Micheal moves to Rockford Hills). However, this is only implied and you'll never see it even if you play for another 200 hours and beat 100% six times. Marcus says he going to Vinewood to star in movies. He is still controllable, though.
[NOTE: You don't choose the endings. They are just possible endings.] Thoughts below.
Edited by hawkstalker, 09 June 2013 - 12:58 PM.