I too think Higganbotham deserved his own theme. I liked the idea of Avalon being evil. He could have got infected by the negativitron or something. I personally didn't like Eve at all. I think they should have made a leprechaun-like character in Eve's place.
I never trusted Avalon since the very first level. I thought that he would for a fact be evil by the end of the game. Kinda sad he wasn't.