Since OOO has the whole '3' gimmick going on so far I wouldn't be surprised if it turns out to become a three-Rider show, and in fact I'd like to see something akin to Agito as has been talked about before. One mystical Rider, one technological Rider, and one biological Rider (ala Gills) :castlerock:
As Lynxara stated, OOO is a very character-driven show, so if several Riders are introduced and manage to work off of one another well it could be a great development.