Many designers, myself included, have a tendency to add on this cool idea or that to the games core. Sometimes it meshes well but often it’s cumbersome and detracts from the game experience. It’s like putting a giant spoiler and spinning chrome rims on an Aston Martin; it may seem cool at the time but you are destroying a thing of beauty.
— Jesse Catron (@ktronod)

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

Cardboard Edison’s Favorite Tips & Resources – July 2012