Well, some of my ideas have been written before, but I post them anyway.
-Missions in Liberty City, Vice City, San Andreas and London(?). You would need to take planes (or cruise ships) like in SA to get there (or fly for ten hours)
-Free roam around all of the cities. Visit old memories in Liberty and Vice City, or take a look at the entire state of San Andreas.
-Cameo appearances. Meet Tommy Vercetti and do jobs for him, give Joey Leone a hand rebuilding the Mafia, team up with CJ to handle some s***, and work for lots of new persons
-Build up your own gang. With the ability to "create" the look of your gang and their wheels. Create companies like a cruise ship firm, build casinos and fronts for your criminal organisation.
-Deal with armored equipment using Rocket Launchers, Miniguns, M60s and Grenade Launchers, or by really big stationary guns like the ones on a warship anchored nearby or the AA-guns on the military bases.