I know this conversation happened waaaay earlier, but now that I'm home from work, I'd like to throw my opinion into the ring:
Do Mii Costumes, Assist Trophies, and Spirits deconfirm? In my humble opinion: Yes, Yes, and and depends on the timing.
There ya go, there's the post.
...Okay, in all sincerity, I believe things like Mii Costumes and Spirits depend on the timing: Rex was most likely only in consideration for Sakurai and Nintendo because the Rex costume's inclusion was more than likely decided pretty early on in development as an incentive to purchase FP1. When negotiations for FP2 started, it gave them the opportunity to look back at Rex and XB2 and go "Huh, y'know, that game was popular and there was a pretty vocal outcry against the Mii Costume. Maybe Rex would be a good fit, now that the timing is right". However, in the end, Pyra and Mythra were chosen because of the technical complications trying to work Pyra in with Rex would bring.
Now, does this mean every Mii Costume ever is back on the table? Not really, no. Rex was under a very specific and fortunate circumstance that, ironically enough, didn't even benefit him in the end. Mii Costumes added in a currently ongoing pass most likely will not feature as playable characters in said same pass in my opinion, as I do not see the business sense in that: If you already had content coming in the form of a playable fighter, why would you include said fighter as a costume for another fighter if said character was coming in the imminent future? The logic that it would "fake fans out" isn't very convincing to me, because I honest to goodness doubt Nintendo marketing executives care about messing around with communities like us that, when compared to the actual install base of Smash, is microscopic and not at all on their larger radar. The speculation scene is but a blip in the bigger blue water market ocean: Our opinions and speculation really don't impact or matter as much as a few of us would like to believe, so no, I don't buy it would be done as a specific way to reveal a character via fake-outs and mysteries. Nintendo doesn't operate like that: They are super clear and to the point when they are selling you something. Nintendo wouldn't sell you a chocolate cake and then go "Surprise, it's vanilla!", they'd sell you the chocolate cake and then give you vanilla frosting as a compliment. Would certain characters become viable again if a Pass 3 was announced? I wouldn't see a reason to say no, as there wouldn't be any good reason why a costume considered and planned for a previous project plan would impact a completely brand new one, but that isn't the situation we're dealing with here with the characters we're discussing and the most likely scenario Smash Ultimate is sunsetting with FP11.
Spirits have been talked to death and I don't think there's much more I can add about them myself. I think what I wrote about Mii Costumes can be applied here as well: It depends on the timing. Base game Spirits? Sure, they're in the running: Min Min and Pyra prove that! Post-game events? That's where I become doubtful.
Assist Trophies are a pie-in-the-sky want from this fandom. Look, believe me, I get it: I'm a huge Waluigi supporter, I'm bummed he got snubbed again too, but the reason so many people treat it like a deconfirm and caution hyping and tin-foil hatting is because we have multiple pieces of information from Sakurai himself and from the way ATs work period that imply that Nintendo considers that representation enough. Could that be broken? Anything is possible, but all I'll say is that there isn't much evidence to show that it will, meaning I personally wouldn't expect it and I would advise responsible hyping.
Also, since this topic also came up: Don't be so concerned about being "right" or "wrong" about Smash speculation: Just have fun with it! Most people here are here because of the community aspect and comradery that comes with have mutual interests and passions. If you're getting hung up on speculating for brownie points and "LOOK AT ME, I'M SO SMART" attitudes, then I don't think you really understand the point of why most are here, and I'd really advise against it. It kinda kills the fun.
