My thoughts when planing my arena was simply that 3' circle could translate to a 4'x2' oval or expanded circle without losing too much of the playing surface.
If it turns out to be an issue when it comes to starting a game then I can start each team at one end on the field .
Also bearing in mind that the game isn't grid based and as such movements aren't limited by direction other than backwards when pushed, the shape of arena will allow for some tactical decisions in the same way that traps and features will.
I can see a variety of shapes adding to the game.
It might also be great to see how the game works in a maze like structure with narrow passages and pits and traps around every corner. With gladiators protecting a stretch of corridor and a single player trying to reach freedom in the centre.
But that may require an expansion pack or set of house rules.