Engine-wise, the outer planes would be easiest to do (basically, they are just like somewhat more/less hospitable Prime Material Planes); the etheral and astral have their own funky rules, which we may/may not be able to implement. The worst are the inner planes (except maybe air)! Demi-planes will depend....
Be sure to include the Positive and Negative Material Planes (for a quick party demise?)...
