Something that I just remembered: the Vs version of SMB1 includes some levels from SMB2, as does the All Night Nippon version (and in the latter case it already had the graphics facelift that SMB2 had). Starting to wonder if SMB2J was just a gradual change of what a "more complete" SMB1 would have been.
And on that note, if back in the day they had the chance to send that kind of updates for SMB1 they probably would have, considering the existence of those upgraded versions I just mentioned.
EDIT: wait, All Night Nippon also has the extra worlds from SMB2J (albeit changed a bit around and some levels swapped with earlier ones in SMB2J). All these games came between 1985 and 1986 so it's really sounding like they just kept iterating on SMB1 non-stop during this time. Anybody wants to try making a list of how it evolved?
It shares a lot more than Link's model, but then again it literally was originally intended as a mod of OOT (back when they were planned for the 64DD, presumably with OOT as a cartridge and Majora's Mask as a disk). The gimmicks around it completely change the overall way it plays though.

