I'd be in favor of a 3 country/ 1 base each setup with some detailed terrain (bridges, taller trees, buildings, hills, valleys, rivers, lakes).
I also think the previously mentioned 2 countries, 3 base idea with all planes in the middle and A v A in the outer bases. As the plane set expands, you could put EW planes on one end and LW planes far away on the other.
I personally don't think the 3 country / 6 base setup we have works that well for 40-50 players. What I saw last night was Rooks v bish at the lower two bases, almost no knights and nothing going on anywhere else. EBM switched to Knights and it was almost exactly even in the arena but the fight stayed mostly rook v bish at the lower base. We ended up switching some of the squad to bish to get a bish/knight fight going. It picked up a bit after that.
Nice thing is we have 4 arenas. It would be easy to simultaneously try out a few options.