Well, i'd say something like that:
1. A more beautiful city.
LC had only one true good lookin' borough, and that was Algonquin. I wish that the entire city would be like that. I was pretty much reluctant to go to a borough that wasn't Algonquin in IV.
2. Adding some SA features, while maintaining IV's.
Although I'm a fan of IV rather than SA, i believe that the formula of TBOGT was the ultimate one. IV & SA combined.
I don't want that stamina thing, or going to a gym. But, i would prefer having the martial arts centers, the parachutes ( of course ), transfender with a much more diverse selection, gambling, really action packed missions, planes if the setting is big enough, golf, air hockey, etc...
But they should also include a video editor too, it adds so much replay value to the game.
3. Graphics.
I like a lot the RAGE engine, but for some reason, i hate it when for example, that in indoors there are clean visuals, and only there. I'd rather having that at the outdoors as well.
Well, that's pretty much it.