A tourney for only brown belts or only 3rd dan black belts will always be empty, as you see in the black belt Aikido room.

Seems like a good idea to add tourneys like white to green belt (50TC), blue to brown (250TC), and black belt (500TC) tourneys.
Lowering the prize for low belted players could avoid black belts to make an alternative account and own white belts for farming TC.