Don't follow the Saint's Row series (but do give us co-op)
With San Andreas, it could be argued that Rockstar was a victim of its own success. The advances made with the last PS2-era GTA were so popular that many gamers felt they were missing out when Grand Theft Auto 4 took a more serious tone.
If it does turn out that the next game will once again be set in San Andreas, we'd hope that Rockstar would resist the urge to simply recreate its world from the 2004 game. Yes, everyone loved the fact that you could own an airfield, break into Area 51 and steal a jetpack and the like, but there's already a sandbox gangster series that delivers this kind of silliness: Saint's Row. Indeed, with its dildo-swords, bobblehead costumes and private jump-jets, it looks as if Saints Row the Third has already pushed the shtoopid factor as hard as it can go.
On the other hand, we'd definitely like to see GTA 5 borrowing Saints Row's support for co-op. Even if it's just a case of giving the second player a 'lackey" character (as per Fable 2), or else cloning the main character (as per most Capcom games), it would be a massive plus if you could buddy-up to work through the campaign with a friend. Aside from the fact that Niko's phone never stopped ringing, GTA 4's mate-dates felt lacking because you were going on them with the AI. No-one would have made fun of the bowling if they'd been able to go with a human chum.