I'm in the process of creating a small(ish) roller-coaster next to the arena, just putting the skeleton/frame up now, then need to get some more iron and start crafting tracks..
I also plan to try and make a boat race type thing with water currents in the future.
Perhaps a water slide/drop.
Since the portal has been put in (thanks Akira) and it's easier to travel there I thought I'd make the site in to a kind of playground/amusement park if theres no objections. Theres nothing else around for miles and the terrain is that grassy/plains type stuff so I'm not really ruining any scenic vistas or anything.
If anyone wants to travel to the site and put up their own fun to play with structure your more than welcome, add to the roller-coaster, go nuts. I see this space as more of a community area than "mine".
Also toying with the idea of another arena where folks can PvP each other and maybe something simpler like a snowball fighting area
After seeing the vid Sor posted awhile back, might be fun to try and make the Rock 'em Sock 'em Robots thing too, I would need some help from some more redstone savvy people for that one though.
All of this just plans for the space at the moment, and if no-one objects. I'd like to hear any suggestions you guys have too if you'd like to see something else.