So after Kazuya, one fan rule is partially dead, two lines of speculation have been strengthened, and we've got a third character that I like to classify as a second choice fighter.
Partially Dead Fan Rule: Mii costumes disconfirm
It's still the case for specific characters but the Mii costumes don't disconfirm the series or the company they are attached to. This means we could, at least in theory still see a Fallout character, an Undertale character, Rayman or Mugman.
Strengthened Speculation Line 1: VA theory
The theory is that Smash DLC does not have the money to pay for an international voice cast so any new characters would either only have two set of voices, use recycled lines or have none at all. This has been true of every fighter so far with Pyra and Mythra being the only ones with English and Japanese, Sephiroth and Kazuya being solely Japanese, Min Min speaking Chinese in all languages and Steve not even having his famous OOF! This line of speculation mainly hurts characters from existing series that are already multi-lingual such as Pokémon and Sonic
Strengthened Speculation Line 2: Runner up Choices
With a Tekken character getting in, it lines up with the idea that Pass 2 is made of fighters that missed the boat the first time around. ARMS and Xenoblade 2 were too late for the base game, Minecraft was apparently in five years of negotiation, Sephiroth appeared to have brought the rest of the Final Fantasy 7 representation they wished to bring to the game and Sakurai had already spoken about wanting to include a Tekken character but having moveset difficulty. Fighters that have been discussed as having been considered for Smash before being dropped go back a long way but between Ultimate and Smash 4, include Rhythm Heaven characters, Decidueye and possibly more we don't know about.
Second Choice Fighters?
ARMS, Xenoblade and Tekken all had one thing in common in that the perceived front runner character (Spring Man, Rex and Heihachi) was already tied into Smash in some significant way with Spring Man being an assist trophy and the other two being paid DLC costumes. All apparent replacements are major characters in their own right with Min Min being part of the fighters ensemble, Pyra/Mythra being the actual focus of the plot and Kazuya being the series first protagonist and first twist villain.
