The problem here is managing expectations. Hard core fans say they want the same game again, but in reality they don't. Because that ultimately becomes boring. But if you revamp everything, then why bother making a sequel and not just an original IP ?
Movies have faced this problem since forever. Too similar, it's pointless. Too different and you lose the fan base. Know many movies where the sequel was better than the original ? Me neither, beyond the often quoted Star Wars and Godfather examples.
In the past game sequels got by on technological advancements. The game you liked before but with better graphics, music, AI, whatever. But as technological quality is starting to plateau and design is coming to the fore, this is likely to be less of a selling point.
Ultimately sequels fall into the trap you've already described. I think the 2nd iteration of a series can be the best if you take a great idea in the original and then polish and refine it in the second. But then after that, it gets really hard to keep things fresh. Unless you totally re-invent everything from the ground on up, like the Final Fantasy series.
An interesing game to keep an eye on for this will be Dragon Age 2, which looks like it's totally changed everything from the original except for the world its based in.