While the idea of having Demigods leave when a player concedes is all right in theory, it will just end up with a one-sided and unsatisfactory victory. Regardless of how many DGs are left on the losing team, there are still towers to push, and without any resistance that is pretty unexciting. My idea is a simple fix to the quitting problem.
Citadel Self Destruct:
It's a 3v3 situation on Cataract. Oak pushed all the way to the middle. The Rook is holding the left portal and Sedna is holding your right one. The game is over, but it will last about five to ten more minutes before the citadel is finally down. Instead of quitting, the option to destory your own citadel is available. It will take a 100% majority vote, everyone must vote yes. When a player does drop it counts as a vote for yes. With a giant crack of thunder from the Finger of God, your citadel is destoryed and the match is over.
This will work on all gameplay modes, and solve the AI player problem.
What do you think?