What do you think?

Setup:
1. No board.
2. Every investigator draws starting items.
3. Random Ancient One (replace AO if his attack requires trophies)
4. Fight it (Epic Battle Variant)
5. See how close you be to kill him (who knows, you might even succeed)
Time of play: Approx. 10 minutes.

Could this be fun? I thought of that because of my games end by sealing victory and I almost never get to final battle, so I feel I didn't experiece the AO to it's fullest. Anyway what say you?

Nah. The point of Arkham is to prevent the AO from coming to town, using Final Combat only as a last resort. Part of the game then may include acquiring items to help or getting penalties to hurt your chances in final battle. It might be fun to try in theory but it's certainly not going to be a satisfactory experience in the end.

I would be far more interested in trying to fine-tune an "expansion board only" game.

I use this method to test ancient one attacks on custom GOOs

Actually, a series of Ancient One only battles with a character and item selection method would be a fun variant (for someone who was into that sort of thing). I have no interest in designing it though.