You know, I was thinking. If Isaac is in the 2nd Pass, do you think part of that reason is for the letters we sent to Nintendo last year? That could have been the turning point there. IDK.
My response letter from NOA is dated March 3, 2019. We can assume they passed on the feedback at or before that date.
The only other date we have is that in November 2019, Sakurai recorded Byleth's video stating that Nintendo had decided upon all fighter's pass 2 characters already.
According to Hiroyuki Takahashi saying he left Aces before it released, pre-planning/early development for a possible GS4 started early 2018. FP1 was decided on around the early summer of 2018, according to people at Microsoft who were involved with Banjo and Kazooie's deal. Then, then of course, the events of summer/fall 2018 Smash speculation were a large part of the Golden Sun hype.
Just my speculation from here out, but it looks like it takes a year or so to get a Smash character from concept to released. Probably a bit less if it's an internal Nintendo character, or if the communication channels are good. So ya, I absolutely think our feedback had some effect and I think if/when it does end up happening, you'll see some acknowledgement of our efforts.
But along the same lines, a modern game takes substantially more than a year to be made. Then basic marketing 101 (and the absence of the Metroid Prime Trilogy on Switch) is going to tell you that you want to make some synergy with your announcements. I still think about the lack of any new GS music in Ultimate when Sakuraba did 5+ remixes for the game...
e.g.
2018:
"What about Golden Sun for DLC? Isaac keeps popping up in polls and requests."
"Camelot's still working on the Switch game, do Fire Emblem instead that'll be out 2019."
"Yeah that makes sense."
Sakurai: *sweats*
2019:
"Dang people still want Golden Sun look at all these letters and tweets."
"Cool, easy Smash DLC for 2020/2021. Make sure it comes out around the game."