Underground would be cool if the tree has a river or something round it, or scenery with smaller trees and flowers especially with the new flowers in 1.7 It would take a lot more time and effort though, if you ever need help I'd be happy to give you a hand
This morning when i woke up i thought of your idea to! i was thinking of a nice big tree with lanterns going down the leaves! also it would be nice if you could help me as thats a lot of dirt to dig for 1 huge tree and little trees in there