I'd like to recommend you saveourearth because it hits all your preferences. It's an AU/mem-loss game (characters get reincarnated into modern day Earth and can then regain their powers) that probably falls into the adventure genre, if I would have to apply a genre. It has a ten comment AC, is on DW, and the setting/events encourages both SoL and metaplot participation, it's up to the players what they do with it. The game is slow and backtag-heavy, but pretty steadily moving. We don't have any castmates, the closest we come is someone from a different FF.