I had this thought before and with your speculation + what you have heard at the dinner table, I could see the 2 more "passes" filled out like this:
-The first pass is Sakurai's Picks. Mainly filling out the "history" of video games in Ultimate. As others have said, Banjo is kind of an exception, but Rare was a big part of Nintendo's history.
-The next batch of fighters would be The Echoe's Pass. Five echo fighters to fill out some of the 16 slots without too much work from the team. Shadow, Black Shadow, Protoman come to mind.
-Then the other would be The Fan's Pass/ Spirit's Pass. Basically, what we, the fans, would like to see. Could easily see the fighters being a combination of Geno, BWD, Rayman, Shantae, Rex & Pyra, Isaac, Waluigi and/or whoever.
I know i am late jumping on this, but you guys did a lot while i was asleep.
I can see an echos pass then a spirits pass. Though the echos pass should be cheaper, since they are exhos. And hopefully they are more like roy to chrom or pit to dark pit echos then just peach to daisy echos. So they are a bit different. Maybe also release them sooner, like 1 a month then start the spirits pass.
If we were to go that route, here are my suggestions.
1. Medusa(Paultena echo) we have no female villains, and Medusa is a bery popular villain. Plus sakurai gets on of his characters in as well.
2. Ms Pac man(Pac Man echo). Syill dont get why tbis wasnt in the base game. Namco bandai deserves another rep and while an echo is not the best it fots, plus ms pac man is quite famous.
3. Impa(Shiek echo) people have been wanting another zelda rep, and Impa is the last reoccurring character in the zelda franchise. Give her eother her skyward sword look, or her warriors look, or both as alts.
4. Black Shadow (Captain Falcon Echo) F zero does need more reps, and this would also be a good way to bring back a lost playstyle. Melee ganondorf os not in the game anymore, so this could bring that style back. Maybe when they do this they also tweek ganondorf as well, and change his down b to be the tennis energy ball attack. Also make it that using the down b in the air lets him float up a bit as he fores it, to assist in his recovery and make him more different from the black shadow echo.
5. Octoling(Inkling echo) i would say shadow for sonic, but we most likely are not getting any at upgrades at all. Splatoon is nintendos newest big franchise so them getting another rep works, and they would be a very simular pit to dark pit style, slower attacks but hit harder.
As for the spirits upgrade pass, here is how i would go it.
1. Shantae. My most wanted, and the biggest indie character to pop in. Would make a great rep for indies amd just in gameplay in general.
2. Andy/Sami. Advanced wars is an underused series on Nintendos end and upgrading their spirit would be great for them. They would also be a very unique gameplay style with maybe a pet based control style with the tanks and fighters they can summon.
3. Rex and Pyra. We know that if their gsme was slightly sooner they would have been in, so this sounds like the best time to add them.
4. Rhythm Girl. I know people think chorus kids are the best but she could incorporate them and more with her style, as she is the mascot of the series basically.
5. Geno. Best way to end the pass is with the big boy geno