Housing will be a pretty large part of this I think, but I agree; Things like Bestiary's, Card games and Chocobo Raising which will let you log in and get a deeper understanding of the world is a good idea.

Immersive content is important in the longevity of an MMO.

I think a lot of people see small things and think that they aren't important, but in an MMO God is in the details. You'll remember the little things. It's what pushes a good MMO to be a great MMO.