Roppongi was originally called 材木町 (Zaimokuchou) or "wood material area"
I think the name Roppongi is a small leap from that.
Actually Zaimokucho was just the small area of Roppongi where Tere-asa Dori intersects Roppongi Dori.
I always like Akihabara (Field of Autumn Leaves). BTW it used to be pronounced Akibahara, which IMHO is much more mellifluous.