I agree it would be great to see more combat at high gcl, but as someone who is still writing his first fully automated script, this sounds like it could be pretty hard on the noobs.
I know we have novice and respawn areas, but making defense harder just makes it more difficult to set up a stable, long-term colony. Personally I think this would be rough on players with less developed defense code.
That said, maybe there's a way to make defense harder on high level players without hurting lower levels. The introduction of the executor and commander creeps could help with this, since undeveloped players are unlikely to have power enabled in their rooms.