I feel like an RHG-esque deal wouldn't work out unless everyone was really, REALLY, devoted; the DarkDemon FFA had a great start, but it fell to shambles later on.
Probably something pretty simple would be best fitting, something simple and quick like the Doors Collab, Run Collab, Beginners Joint (okay, maybe not this, but it worked out quite well), etc.
Just a random idea: An adoraball is introduced into the field with the default figure. The adoraball has to engage the default in combat and has to beat it up, get beat up, or something. Afterwards, the adoraball exits in some form.