The characters are definitely locked and are being worked on right now. Unless something crazy comes in that would make a character impossible to add (like a crazy big drama over their game or the character which would ternish nintendo's image) they won't change a character. At most would they add a costume to a character based on fan demand of another character from that franchise (like a felix costume for isaac, potentially).
We did everything we could this past year and half, and they most likely noticed it. I don't know how other communities fared honestly. I didn't see events are ambitious as golden sunday. But I may just have missed them, if so, it puts GSunday into perspective because it could easily have been missed by people too.
On the other hand Golden Sun had a massive resurgence in gaming discussion this past year (completionnist, Corey Barlog, a few french tubers, Gnoggin a few years ago...) and the trademark is here and got trending, which Nintendo would most likely notice.
All and all, that second pass bodes well for Isaac. Timingwise, it was decided around the time when Golden Sun got a huge resurgence in popularity (even if we don't know the exact time it was finalized, probably around november since the direct seems to have been recorded around then from what I heard) it's not bad at all and possibly made it to the consideration table. If it was decided a bit later, maybe Corey's tweet would have had some influence too ! xD
Anyway, I'm quite confident about Golden Sun's chances in that second pass. We've got 6 shots, and I just hope we won't have to wait until december 2021, when the game is already 3 years old, for isaac to get in. It'd be terrible to promote a new game/franchise, since it would be when the interrest starts to die off.