Mine would be a few that would be tough to choose from as a favorite, A casino/gambling update (which may happen), the ability for higher level players to create missions to give to lower level players, and decide the payout and rp based on difficulty (within reason so as not to be abused) And as cliche as it sounds I love zombies and would love a zombie apocalypse lobby.
Are you sure you want to go to red alert? it does mean changing the bulb,
I want a whole area that's similar to the world of The Road Warrior. People always say "This is GTA not _____" but I think this is the perfect game to implement random crap from major franchises and stuff to just screw around with. It's fun.
I want Godzilla. Terminators Predators More Aliens Red Dawn Zombies Robocop
Etc. Endless list.
Oh and hang gliders.
We are the future history. Let's make this past mean something more. http://i.imgur.com/zigV7YZ.gif http://i.imgur.com/Jr84ISK.jpg
More aesthetic customization's for vehicle's. Chassis colors, bodywork, different roll bars, working lights with the roll bars, different color head light's, variety of color's of tint, as well as tail light alterations.
Mansion's! With 20 car garage's. Make the garage something customizable. Have the placement of the vehicle's to be more "free roam" so that you can organize the placement of your cars the way you want them.
Yacht's with a helipad and scuba gear. This should include a designed interior with seating for the entire lobby. Have a high armor and heath attached to the Yacht so that it is highly difficult to blow up. This way you can have Yacht battle's 8v8. Defector's can fly over to the other Yacht or assassin can scuba in from underneath. This would need to include a jet ski boarding bay as well as a lock so that you can take a couple of jet ski's on the back with.