A sequel is usually done from something that has already been successful.
Successful MMOs can carry on for years and years and it's no big suprise that we don't have too many MMO sequels.
The main issue i see is taking away subscribers from the original game, as mentioned in the article.
EQ1 to EQ2 is a good example.
Take an established world and set your sequel at a different point in the history.
Don't have the exact same locations so that people get to see new area content.
Expand the original game content with also adding in new classes, races, crafts and other new game mechanics.
Improve the graphics so it looks up to date.
Over time with expansions, add in areas from the original game world so the players from the first game get their nostalgia fix.
EQ2 had many issues for the first year, but was revived back from the dead when the devs realised they didn't want to play the game as it wasn't fun anymore.
i'd like a MMO sequel to consider a path similar to City of Villains.
Have the game similar in many ways to the original, but set appart so that the classes and experiences are just different enough that it isn't quite the same. Also having a link in some way between the original and sequel worlds would be awesome.
-roob