I believe there should be no need for rewards as it would be pretty fun how it is. For individuals to not get super speedy XP by 1 hitting noobs all day you might get a smaller amount of XP gained (similar to battling random events).
There could be three arenas: Standard combat. I know I haven't got to the specifics or precisely how it would all work, but for now all I'm looking is a feed back for this idea to expand. I don't think it could be too difficult for geniouses like Jagex to create since it's predicated on FoG, Clan Wars, Deul Arena, Castle wars, as well as jungle. So what exactly do you think?