The OA feat sounds nice, but I wanted to limit it to Core Rules, because once you go further afield, who knows where you'll end up? Netbook feats?
It seemed to me that taking a BAB-20 class was more workable than taking a BAB-15 or BAB-10, because it does make a difference with Power Attack.
A BAB-0 multiclass gets one attack at +0. A 20th level wizard using full power attack gets two iterative attacks, at +0 and -5. A 20th level fighter gets 4 iterative attacks, at +0, -5, -10, and -15. It's that -15 that I'm after.
Attacking through a doorway grants cover to your opponent, not an attack penalty.
-Hyp.