The biggest issue the Arena has on RPGuild is that it has such a small pool of players. There's probably sixteen active players -- twenty, max, and that's with the newest influx. Teams might be helpful, but I think the somewhat-anemic population would need some bolstering.
The second issue, which dovetails with the first, is that there's no coherency. It's somewhat adrift. There's a vague "setting" in the background, but that's just to explain why and where the fights are happening. There's no connection to any outside roleplaying. The fights are just kind of adrift, free-floating. There's no anchor, nowhere for the characters who fight in the event to really lay their roots. I think there was an attempt at this on oldguild, but I'm not sure how successful it was. There's no roleplay threads - cities, bars, locations of any kind - where character who participate in fights can do something other than fighting.
It doesn't help that RPGuild's organization is based largely around how a person roleplays, rather than what they're roleplaying. It's a bit sixes and sevens - on one hand, you have the casual/free/advanced sections, then you have themed sections like nation/arena/1x1. It's like someone changed their mind halfway through. This also makes it difficult to know where to place a thread like, say, a city that acts as part of a "persistent world" where people play.
I think that's part of why it's so hard to draw people into the Arena. In chatrooms, you always had a persistent world.. even if that world consisted of approximately 100 taverns, bars, inns, and castles. On Gaia, you have something similar: in between fights and tournaments, you character can go dick about in the city of Durem or hang around a bar or.. whatever. There's some kind of continuity to your character's life outside the fights that can be roleplayed, and the fights become part of that character's story.
For once, though, I'm completely fucking knackered for ideas on how to fix that, because it seems like RPGuild's structure isn't going to make it an easy fix.