According to CVG's recent GTA IV Collector's Edition magazine, the Xbox 360's downloadable content could be even bigger than anyone has made it out to be. It could be a whole new city.
Of course, to call games as vastly ambitious as Vice City or San Andreas mere 'expansion packs' seems childish, but nevertheless, the downloadable content (DLC) coming for the Xbox 360 version of IV has repositioned those games in just this way. DLC so far has meant the odd new car, jumper of bit of horse armour...
...but GTA IV is set to completely redefine the idea with expansions that are to GTA IV what Vice City or San Andreas were GTA III. Yes, Rockstar is clearly hinting at new downloadable cities; and the chances of them being London, Vice City or SA again are slim to none. So that's new as in brand new. GTA IV's Liberty City is the beginning. Think about that and be excited.
The only problem we see with this scenario is that new cities can't just be generated with the flip of a switch. They require meticulous, intelligent design so they stream appropriately, and a ton of playtesting to sort the bugs.
So while it's certainly not impossible for Rockstar to cook up more cities for GTA IV's expansion packs, it's a difficult enough proposition that this rumour comes off as a bit tough to swallow (though, hey, we sure do hope that it's true).