As far as I'm concerned, this is the greatest game of the last generation. Not my favorite, mind (it's in third place behind Oblivion and New Vegas), but far and away the best. Apart from falling into some of Rockstar's usual narrative traps, I find it hard to fault anything it does. The atmosphere, worldbuilding, side content, detail, sense of closure and interaction is spot-fucking-on.
As for the next game, while I don't really trust Rockstar to pull off the narrative in a particularly nuanced and sensitive fashion, I think a Django Unchained-style tale set in the slavery-era Deep South would be an awesome way to differentiate itself from the typical Old West setting and distance itself from RDR.