So, to map things out, the following 3rd party character costumes weren't in the Smash Ultimate base game:
-Tails
-Knuckles
-Chocobo Hat
-Geno
-Zero
-Megaman X
-Protoman
-Megaman Exe
-Akira
-Jacky
-Heihachi
-Gil
-Lloyd
-Monster Hunter
-Rathalos
Following the Power of Six Theory, every Fighter Pass was originally supposed to have 6 Mii costumes total with at least two returning Mii costumes per pack. 15 costumes and 5 passes, making an average of 3. So, let's map this out further:
Joker- Knuckles and Tails (2)
Hero - Geno and Chocobo (2)
Two passes with only two so in order to reach the final 15, we need to fill 11 more spaces across 3 passes so two would have to have four.
Let's navigate the easiest first:
Terry- Akira, Jacky, and Heihachi (3)
All from fighting games so it naturally fits the theme. Now let's go to Banjo.
Given our original theory, Banjo would have to have four. Banjo already has two (Zero and Proto Man) and, conveniently enough, there's two other Megaman Mii costumes (X and EXE).
Banjo- Zero, Protoman, X, and Exe (4)
Thematically they go together. My personal theory is that Rocket Grunt was a replacement because it's the only first party Mii costume in DLC so far.
Now we got to 5 and dump out the remaining 4:
5- Lloyd, Gil, Rathalos, and Monster Hunter
All sword fighters and three have armor and helmets. Seems deliberate. Now let's slam this whole goddamn pizza back together.
Joker- Knuckles and Tails (2)
Hero- Geno and Chocobo (2)
Banjo- Zero, Protoman, X, and Exe (4)
Terry- Akira, Jacky, and Heihachi (3)
5- Lloyd, Gil, Rathalos, and Monster Hunter (4)
15 total
This was most likely the original plan when the Fighter's Pass was one and done, however, things have definitely changed.
Joker- Knuckles and Tails (2)
Hero- Wow, it's ****in' nothing!
Banjo- Zero and Protoman (2)
Terry- Jacky and Akira (2)
9 costumes are totally MIA at the moment:
-Geno
-Chocobo
-Heihachi
-Exe
-X
-Gil
-Lloyd
-Rathalos
-Monster Hunter
What's interesting is that based on this model, Nintendo is intentionally bringing back two Smash 4 costumes with each pass. What does this mean for 5? It suggests the pairing would either be Rathalos and Monster Hunter or Gil and Lloyd. Why? Because all the pairings have been from the same company and, in the grand scheme of things, Heihachi is the odd man out. If Heihachi was "saved", what does this mean? It heavily suggests him either being playable later or saved for a different pass. Either way, to keep this pairing theme going, Lloyd and Gil need to be "saved" too so that either him or a totally mysterious and absolutely unknown Namco have a pairing later.
Heihachi- Lloyd and Gil
Or
Super Mystery Character 64- Heihachi and Gil
Circling back, this means that under this theory that Monster Hunter and Rathalos will likely be the Mii costume pairing for #5.
With that settled, that then leaves 7 Mii costumes. On most basic terms, this suggests that "Season 2" is at least 4 characters if this pattern of pairing continues. On that note, on the event of prrrrrrromotion, it's potentially possible for characters' respective Mii costumes to return too and seems like a possibility anyways. So, with that outline established, we would need suitable characters to "bring" the following character pairs:
-Geno and Chocobo
-Gil and Lloyd
-Exe and X
-Heihachi and, his favorite means of cooking, a Hibachi
So, all in all, fun stuff to digest on the way to #5 and beyond.