Why not do both? The Arena Alpha won't take long at all, then you can move onto koth, which'll take a little longer alpha dev wise... That way, you please both sides of the arguement.
Mind you, I'm saying this without playing it.
EDIT: ran through it... Yea, you should keep it arena for now. Something you might want to consider to up the "fun" level, is to activate the cap sooner than 60s. Backlot activates in ~30-35s. Doing this, there is a slight shift in game flow, from "Beat the shit out of everyone" to "Beat the shit out of everyone while trying to hold the point"... which, when there is an objective to beating the shit out of everyone, things become more fun.
So, as the "resident arena expert" here... I'd say, stick to arena, then convert it to KotH and have 2 versions. People just hate arena, and rather play KotH. But is Arena is what you want to make, then make it.
Also, the front yard where the death-cliff-o-doom is, make that wider, that is a good flank, it is going to be dominated by Soldiers, Demos, heavys and pyros