1. The social part looks interesting if you can flesh it out. I like the bribing aspect, but you will have to balance the bribe amount with the effect gained (or maybe there is a disadvantage to bribing!). Favor looks interesting.
2. Realistically, height advantage makes things harder for both combatants (although in military strategy it may make sense). What would be cool is putting bows and arrows in the center and increasing movement cost to move through the center

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3. Scattering items around the arena will only be interesting if there is some sort of time pressure, either through something like a food clock, time limit, or something else. You want to make a cost for going around and getting the items. I would have a penalty for stalling (they want blood!).
4. I don't like the idea of the reflex meter. I don't want to feel penalized for not button mashing or acting quickly if this isn't a real-time game.
5. Defense through sustains is good, but I would put a price on most of the sustains to make them interesting.
Other than that, I would suggest adding the combat system/arena generation/items in first, and then the other aspects if you have time. Good luck.