Some principles for creating an expansion, from Scott Caputo (designer of Kachina/Völuspá):

1. It has to be desirable.
2. It has to feel like part of the original game.
3. It cannot make the game feel overly long.
4. The new [components] have to work well together.
5. It needs to allow for more planning.

http://boardgamegeek.com/blogpost/12929/designer-diary-voluspa-or-how-a-native-american-su

SEE ALSO:

Lessons learned designing an expansion, by Grant Rodiek (@HerrohGrant):

http://cardboardedison.tumblr.com/post/19177839103/lessons-learned-designing-an-expansion-by