There are still a lot of third-party series that are not in Smash at all and for some of those that are, Mii costumes acted as consolation prizes for them.
Agreed, but that's exactly why it's not a case of "playable or nothing" for big series, especially ones where the company is already involved in a playable capacity. There are a lot of big series that only wound up as costumes or spirits or ATs.
Do you mean SNK? Because if so, Ryo is also a Mii costume and Art of Fighting hasn't had a new game in decades either.
No, I meant SMT and SoulCalibur. They're also effectively part of a shared universe with their respective playable counterparts. It's not just Keen and Doom. In fact the overlap between those series is often much less oblique.
Also yes Art of Fighting hasn't had a game in forever, but characters from that series and Fatal Fury effectively get to count as dual series characters between their origin series and KoF. Ryo is a staple of KoF and still has plenty of recent appearances.
He's also, fwiw, basically the most overall niche third-party costume we have (taking into account that, unlike characters, some costumes skew heavily Japanese or west, and may be quite niche in the other). So if that's the best parallel, it's not particularly auspicious.
Looking at series like SMT and Soul Calibur and then a series like Commander Keen, I think it's pretty obvious which ones are more likely have a playable representation. Thinking they would all likely get the same treatment doesn't make any sense.
At their ceiling, absolutely it makes no sense. SMT and SoulCalibur could feasibly receive a character. Keen never will. But I never claimed the series were on par with each other.
At the floor, they could receive the same treatment - which would be a reference of some kind within a corresponding pack. I think it's likelier SMT or SoulCalibur receive something a little more substantial like a costume or spirit event (or whatever the spirit equivalent might be going forward), but it's possible it's less. It's possible Keen receives more than just a small reference as well, but I think it's quite unlikely.
Also, you just never know. Some companies may feel that their series should have a playable representation or none at all. You look at Koei Tecmo and Arc System Works and realize that despite having a lot of major series, almost none of them are represented in Smash in anyway. Heck, in Koei Tecmo's case, none of them are. And as for ASW, I don't know the reason for that but I can't picture them looking at characters from major fighting game series being playable and then being satisfied with their own fighting game characters being just spirits or Mii costumes. I'm sure some companies would rather want more than that.
But this is a baseless presumption predicated on a theoretical. It's not a real leg to stand on, there's no evidence it will be the case in this situation.
There's no reason to believe KT or ASW have some dogma of playability or nothing, especially since both have IPs that aren't currently playable: Fatal Frame and Kunio, respectively. You've chosen companies where all they have is supporting content. They don't have a lot probably just because they aren't as big as most of the companies that do, and not everyone is gonna make it. Of all companies to be picky, these two seem very unlikely candidates.
Examples like Activision would be stronger. They actually have nothing, and would plausibly be less amenable to compromise. But that's moot now too.
Plus this argument is incongruent with the series we were actually talking about, given there's no reason to believe Sega/Atlus and
especially Namco would have some "playable or nothing" mentality. If anything, Microsoft, current owner of Keen, would be closer; they had no supporting content from IPs apart from the two which were playable (given acquiring the Bethesda content predated the buyout). But even attributing this either/or to them is ill-founded.
This is just an extension of the constantly disproven logic of looking at something absent and deciding its absence is because it can't happen.