Sequels are tricky. Everybody knows about the law of diminishing returns, and about the likelihood of lightning striking the same place twice: the same magic ingredients might not be there the second time around. On the other hand, the sequel could be virtually identical to its predecessor, in which case things become formulaic and boring. Basically, a sequel has all the problems of a new fic, plus the problem of being distinct enough from the first one to make it fresh and interesting.
If there's a new aspect of the plot or the characters from the first fic which the author wants to explore, then with enough imagination a sequel can be an awesome thing. Otherwise, I just wouldn't bother with a sequel - it's not like this is the movie business, where you can get paid millions to rehash the same plot over and over again. There's nothing worse than a really bad sequel that leaves a sour taste in the mouth every time you think about the classic original...