Well I'm glad to see interest, but four people does not an arena make. We'll need to grab the attentions of other arena goers. 
(Four because Biddz is not two feet away from me as I type this.)

Alright, so:

I've got a few basic ideas, a very diverse world something like Runeterra from league of legends. You know, with many different terrains and locales for combat. Small sub-factions and roving groups of powered beings, as well as large over-arching powerful beings leading powerful factions. I'll have to write those up later.

The way I'm thinking of going about it with characters has to accept all kinds of characters, so people aren't adamantly opposed to entering simply because their favorite character would be excluded.

A maximum of 3 low tiered characters.
A maximum of 2 mid tiered characters.
A maximum of 1 high tiered character.

Per person of course, because everyone loves having multiple characters on the map at the same time. It adds diversity, and of course it's really just fun to have different characters in different situations.

The power regulations on the characters will also have to be both strict and open, so I'll have to review them myself.

I guess I should also get to work on a plot and a map. 

Start with the plot, build the map around it, because that's how you make the map realistic.

Biddz will also help with the writing as well, don't worry about the multiple writers to keep the plot in motion.