I think they should ask the fans who they want to see become s2, s3 etc and then just listen and put them in the next game. surely every fan must know how much other fans love ghost roaster, so I'm sure the staff at tfb would know this too. So why haven't they made series two yet for him? I personally think they could also release new series figures that don't need to be announced and included in the release of a game. What I mean is, just release them like how variants are realise.
Let me know what you guys think of this idea