Fun mode - A mode where everything is free so you don't have to worry about how many times you can shoot a cool weapon or grinding Roof Top Rumble 70 times to buy a good car. It also gives you the ability to manipulate lobby rules like friendly fire, health levels, cops on/off, peds behaviours, etc.
"If you want learn something... learn how to read my friend" WMC128
A mobile, customizable Yacht would be awesome. It doesn't even have to have a separate "home" cell like apartments, just an open yacht like the one in BOGT would make my day.
Hunting Missions. Casual and competitive. Single shot rifles/shotguns only, and custom golf carts to zip around the hunting area. Why don't we have this already?
Co-Op Missions with heavy mission modifiers. In other words, an assassination mission for Madrazzo, but rifles only, sport cars only, stealth only, or a combo of all 3. Even missions that require players to change clothes would be interesting(say a mission that requires you to kill someone that is at a fancy event, and the only way to get in is everyone goes to a clothes store and changes into a Tuxedo)
Casino open, along with lotto tickets for sale at convenience stores, and added radio dj giveaways('6th caller right now wins an exclusive FlyLo T-shirt and $2500!', where the DJ then tells you a random number to call, and everyone really must dial and try to be the 6th caller or so in a full lobby)
Adding 20 new vehicles at once, with numerous being knockoffs of famous vehicles(Batmobile, Kitt car, Delorean, etc), plus more custom options.
Mansions for sale, with customizable interior AND exterior. Choose your furniture and such for inside, and choose from several landscaping options, pool types, and to add a HELIPAD to your property.
Map and Mission Creator instead of crappy Race Creator. If I want to blank out all vehicles and NPCs on the entire map and replace them with zombies or aliens(which are both in-game already) that attack, I should be able to. I would make finding even the crappiest car that still runs and drives feel like an accomplishment. Finding ammo for your pistol would be scarce, and something like a sticky bomb would be a secret. This can change the feel of the entire game in the right persons hands.