I can't realistically see them adding more cities as DLC. The closest thing I've seen to a game doing that is the extra island in Sleeping Dogs, which wasn't exactly huge. Now if Rockstar was to add another city in the size of LS, that would take YEARS to develop.
My major assumption on GTA DLC (judging from trends in other Rockstar game DLC) is that we'll get a whole bunch of game enhancing add-ons. You know, the kind of stuff that won't greatly affect the overall gameplay, but are fun to get your hands on.
We already know from the Pre-Order packs that there'll definitely be extra outfits, tattoos, vehicles, weapons and properties as DLC, which will most likely be bundled together in packs. We'll also most likely get this kind of downloadable content in our multiplayer mode as well (think RDR, Max Payne 3), along with extra game modes.
If we're lucky, we may get one or two mission packs (depending on the success of the game) which will add new interiors and goodies to the game, but keep in mind that these take several months to develop (TLAD and TBOGT took just under a year each, as did Undead Nightmare and LA Noire's extra cases were pre-developed).
Now this is a conservative prediction, Rockstar may throw us a curveball, but I'm just following trends here.