Actually, the Uragaan Streaking challenge does sound pretty good!
Arena: Land Arena
Main Enemy: Uragaan
Other Enemies: Rhenoplos, Bullfangoes (re-spawning of course)
Special requirement: You must defeat Uragaan without wearing any armor or talismans within 50 minutes or until you are defeated thrice. However, you can bring in any weapon and extra items (potions, whetstones, for example) you want with you.
This really will turn into a test to see if you will end up "overfocusing" on the main enemy.
I also had another idea for an Arena challenge:
The Trial of Endurance
Arena: Land Arena
Main Enemies: Basically every large monster that is the quarry of a hunt or capture quest!
Other Enemies: Jaggi, Jaggia, Rhenoplos, Bullfangoes (re-spawning of course)
Time Limit: None
That's right - there would be no time limit on this Arena Challenge. It begins with a Great Jaggi, then a Qurupeco, then a Royal Ludroth, then a Barroth. Next is Gobul, and so on down the line.
The idea is to see how long you last against these monsters, and how many you manage to defeat.