[DGDobrev]

Well, for starters, there is no multiplayer mode. Maybe in the future such a patch may pop up, but I surmise that would need a serious game reworking.

So far there have been rumors for official game additions that will enhance SP (castle sieges, even more items, etc.), but nothing that involves the addition of a second (or more) player. I wonder if hot-seat can be implemented. It will not require creating a multiplayer mode, just make an arena (as you suggest) and allowing both side units to be commanded but player 1 or player 2. In this case the AI will simply take no part. I would readily settle for this:
Bill Gilbert trains Amelie - you select the units for each of them, elect the class and either you play vs. the AI or a friend takes over one of the chars and his armies.

Although I must admit I'd like to see something like this added, I've always found KB series as the ultimate SP experience. The AI is decent, difficulty setup is good, the game allows a multitude of advancement paths, and can be easily modded to improve even further the SP experience - by making the enemies tougher, increase their numbers, etc.