Some principles for creating an expansion, from Scott Caputo (designer of Kachina/Völuspá):
- It has to be desirable.
- It has to feel like part of the original game.
- It cannot make the game feel overly long.
- The new [components] have to work well together.
- It needs to allow for more planning.
Lessons learned designing an expansion, by Grant Rodiek (@HerrohGrant):