Umm.. His west popularity volume in comparison, well mainly US is no secret. That's the only thing not going for him. Call me a fan but I think (I could be wrong) he wasn't an AT because he's too good to be one, he solely fit to be playable, nothing less. You see, like a sidekick or a package, I think 3rd parties come with their own ATs. Raiden with Snask, Alucard with Simon/Richter, Mega Man with Elec Man/Zero/Dr.Wily.. etc.. What proves somehow to me (again, could be hopeful thinking) is that Fatal Frame got to be an AT but not Goemon.. I really think that opens up the possibility for Goemon to be one of those 5 DLCs.. oh sorry, those 3 left XD
None taken. Even if you meant to, it really wouldn't bother me much. Well, there're no more "popular" Konami reps left more than Goemon. Also, as I said in the beginning, "popularity volume". He is popular enough in the west to valid a entry imo, though scattred fans and lack of new games is what makes it look otherwise. Been a port in 2017 SFC mini.. so that's fairly recent.
No character is 'too good' to be an Assist Trophy though. I mean, Isaac, Bomberman, Zero, Knuckles, Shadow, and Alucard are all Assist Trophies and they are all popular, beloved characters that many would say are 'deserving' of being playable.. Raiden didn't come with Snake though, that's Grey Fox, from the first Metal Gear Solid.
Fatal Frame got an Assist Trophy for two main reasons.
1) Not much combat potential in a camera.
2) Nintendo half-owns the entire series from some point onwards.
Bomberman is clearly more popular world-wide then Goemon, again, no offense, yet Bomberman was made an Assist Trophy. I don't think Bomberman would be any more or less 'good' then Goemon in regards to being an Assist Trophy or Playable Character.
In regards to being popular enough in the West..not really, I'd say he was more niche. I mean, I was big into the Nintendo 64 and I barely remember Goemon unless I see his design (which is very distinct and unique, I will say)..and I remember Glover for crying out loud. Its a Cult Classic if anything. I didn't even know that a GameBoy game released at the same time.
And then in regards to not having any popular Konami series left, I'd disagree since we still have Contra, Silent Hill, Suikoden, Zone of the Enders, and Adventure Island, all of which have seen more success in the West then Goemon did.
Would Goemon be a unique and fun fighter? Oh, yes, absolutely. Will he appear as anything more then maybe a Spirit? No, probably not..not when they could add a more popular character from a new or existing series from Konami like the ones listed above or Raiden from Metal Gear or, more likely, another developer entirely since Konami already has 3 characters in the game so I could see them wanting to pick from a different 3rd Party dev with more popular series to include Namco Bandai (Tekken, Tales of), Disney (Kingdom Hearts), Square-Enix (Dragon Quest, NieR, Chrono Trigger), Microsoft (Minecraft, Banjo-Kazooie), Activision (Crash Bandicoot, Spyro), etc.
I just feel that given all of that, Goemon's chances seem much more slim since, for most Western players, he would be a 'literally who' and 'literally what' in regards to his character and series respectively.