I would love it if Rockstar would do this (ok it's hard but it would be perfect).
Make a GTA for what happened before and after (for example: ) GTA 3
Like Claude's early life, including a few missions, and after GTA3 a few missions
Tommy's early life a few missions (we know much about it thou') and a few missions and a few missions after GTA Vice City (like did he die?)
Carl's early life, a few missions, and after GTA SA (we already know he didn't die, but we all would love to know about his mom's and brother's death, and what he did after SA)
Toni's early life, a few missions, and after GTA LCS what happened, and a few missions
Same with Victor and Niko (BTW was he from Croatia or Serbia or...) Johnny (ok we know he died but...) Luis
And a few missions of early life of Michael, Franklin, and Trevor.
WE LOVE ROCKSTAR!