If MH: World keeps the Arena fights from MH3U, then there should be "limitless" arena matches.
Basically, you would have only one life (none of this triple-carting business this time), and the monsters would keep coming randomly one after another until you finally are defeated.
The rewards would be coins for each monster beaten, with the coins doubling each time five monsters are beaten.
An example from MH3U would be:
First, Great Jaggi, then Royal Ludroth, then Rathian, then Qurupeco, then Duramboros, then Rathalos, then a certain monster simply called "The Devil", etc.
I've also posted this to GameFAQs.