To answer some questions...
I don't know how many "bosses" I'll have per release. I might sometimes make a single stage and release for all I know. Every World will only have one "real" boss. Individual stages may or may not have minibosses. Of course I could always have secret bosses and stuff too, I dunno. I haven't "planned" anything. There is no "planned" future releases for Gaiden. I just know I've given my self a platform to make more content whenever the urge strikes me. It might not be anything new until after BE:P or I might take a weekend and maybe a level and throw it out there. Who knows. I don't even know if 3 level is the standard per world or whatever.
The short of it: I'm going to do what feels right at any given time and get it done when I feel like it. There might not be another release for a year. Who knows. I might get struck with an idea and make something in a few months.
But yeah, still sticking with 9est for now, though things are looking good. I'm sure there will be some weird ass bugs after release, so I'm expecting a few versions of me touching things up and adding features throughout the month but I dunno if any of that will include content. I might try and sneak in a SECRET STAGE or something, but no promises. I REAAAAALLY wanna get back to work on BEP, even though doing this was a blast.