I don't really see why a VC remake should be such a massive challenge compared to IV. Graphics will probably be more detailed and better than IV simply because of the agespan between the games leading to more space, but I don't see a VC remake as a challenge "too big" for R* at all.
My guess is Vice city. Maybe set in modern time to surprise the fans or a visit back to the 80's.
I actually think it's a brilliant idea, if VC was to be set in 2010-11. Just because the old Vice City was in the 80's, doesn't mean that other decades can be used. And still be an equallly interesting choice.