Ugh. Sorry to be such a compulsive poster, but I've been play San Andreas for a good eight hours now and ideas just keep popping into my head...
Here's my ideas:
*Instead of pre-selected properties being available for purchase, the game should have houses constantly being bought and sold. For instance, let's say you take over a territory and allow crack dealers and prostitutes to infest the neighborhood. The property value should plummet, and houses will become for sale in the area. The homes can either then be bought by you or eventually they will either be purchased or fall into disrepair. Alternately, you could buy the homes, then clean up the neighborhood and then sell the homes for more than the buying price.
*I think that their should be a maximum amount of homes that you can own at any one time in GTA4. Considering that there were 32 properties availible in the game, I think that the total number you can own should be about 60, which accounts for not only the assorted safe-houses you buy, the places you automatically get from mission progress, but also the random properties you buy as a real-estate tycoon.
*Not that this necessarily fits with the topic, but it is MY topic, so I'll say it: I'd like for their to be some kind of gang fortress place you get during the game. Somewhat reminiscent of Big Smoke's Crack Fortress in San Andreas and the decrepit mansions found on Prawn Island in Vice City...some kind of large, rich-looking house, but completely covered in graffiti. It would have large, concrete walls, and many gang members wandering the halls. Like Vercetti Mansion, but way, way, waaay much more to do there.
Please criticize me for these, and tell me how dumb they are. (Actually, I want tons and tons of praise, but now if you criticize me, you look dumb...)