I still don't see Master Chief. Very violent game with real guns and real bullets that create real blood and real deaths, too much of an XBox mascot, XBox is not a thing in Japan, and I still think he has a style issue with the game, much like Goku, and are not in the spirit of it. Doomguy has a much better chance, deconfirmed or not, due to the demonic and sci-fi aspects, and his lengthy history with Nintendo.
People will say "but what about Bayonetta and Snake?". Well having played those games very recently because of the isolation, Bayonetta is far from being realistic violent. There's never a single human injured, and the blood and kills we see are demons vs angels, meaning actually not as gross as Link diving his sword into Ganon's head. And Bayonetta has little to no sexual connotations (at least in the first game), despite undressing for some kills, there's never a sexual tone to it, more that her suit is made of hair and she needs it to summon the demons. Being halfway through the second game though, that one has more sexual references ( a few jokes here and there, usually not fight related), but I don't know if it was as much of a reference for Smash 4 at the time, since I don't know the timeline.
We've known about FF15 for a long time though, at least the first video about it generated a ton of hype, despite being reworked for years to become what it is now. And Final Fantasy 14 was a huge success, getting up to 18 million users by 2019, so that puts a lot of other games to shame. So whoever thinks Final Fantasy was on the low end, I'm not sure that's quite accurate. It might not have made a character as iconic as Cloud, sure, but it certainly isn't dead.
I think this might be the first person on Smashboards to have never played a videogame before. Otherwise the levels of baffling wrongness can't be explained.
Let's break it down, shall we?
1. (Halo is a) Very violent game.
No it isn't, honestly as far as M-rated franchises go it's pretty tame. Much less violent than Bayonetta and Persona.
2. With real guns and real bullets.
Didn't stop Metal Gear, and if you wanna get technical Bayonetta ("real bullets") and Persona (guns that look real), and besides Halo has many non-realistic guns that can be included. Plus as Gwen mentioned the notion of adding real bullets in videogames doesn't exist, bullets aren't modeled and in a game with Smash's perspective you won't see anything more than muzzle flash.
3. Real deaths
I think aside from like, Punch-Out and Wii Fit, there's not a single franchise in Smash where nobody dies. Killing is an intrinsic part of the videogame experience. Even Nintendo games, which are very family friendly, don't shy away from using death - realistically depicted death - as a plot point.
4. Too much of an Xbox mascot
Yeah, that's the point. That's why he'd be so exciting. Microsoft is already in the game so this is a moot point.
5. Xbox is not a thing in Japan
Xbox is accurately very unpopular over there, but Halo is one of the most successful Western franchises there (I think it's maybe like the third after Crash and Assassin's Creed but don't quote me on that). There is a PC audience there, and lo and behold Halo is on PC. People over there know about Master Chief, and it's not like Halo hasn't made enough of an impact so as to not be considered for being Western.
6. I still think he has a style issue in the game
You phrased this as an opinion so I'll give it a pass but most people would disagree that Green Samus isn't a fit for the game
7. Much like Goku
Goku has literally zero style issues considering we got four characters with that artstyle in Smash. Goku isn't getting in because he isn't a videogame character.
8. Are not in the spirit of it
Dunno what this means
9. Doomguy has much better chance, disconfirmed or not
See here's the thing, if a character is disconfirmed you can't have a worse chance, that's impossible.
10. There's never a single human injured in Bayonetta
I don't know if you're not counting the named characters in Bayonetta as human for whatever reason but there are deaths and injuries. If you're talking background NPCs in cutscenes then I dunno, seems improbable but I can't check it.
11. The blood and kills we see are demons vs angels, meaning actually not as gross as Link diving his sword into Ganon's head.
Uh, what? I'm sorry, but even if you don't find those demons and angels exploding into blood gross, the point isn't how gross it is, it's how gory it is, and it's objectively gorier than anything in Zelda. The design of the angel bosses in Bayo 1 by themselves are gross and disturbing.
But ****, if Zelda is gross and gory, then Master Chief absolutely gets a pass.
12. Bayonetta has little to no sexual connotations, despite undressing for some kills, there's never a sexual tone to it
At this point I'm considering the possibility that you're just a troll bored during self-isolation. You're telling me that Bayonetta, introduced wearing a sexy nun costume that rips in all the right places Bayonetta, pole dancing during attacks Bayonetta, spanking enemies Bayonetta, isn't a game with a sexual tone?
13. more that her suit is made of hair and she needs it to summon the demons
Sure pal, and Quiet needs to breathe through her skin.
14. I don't know if [Bayonetta 2] was as much of a reference for Smash 4 at the time
Bayonetta got in Smash because Nintendo was publishing Bayonetta 2, and her default costume is the Bayo 2 one, so you do the math
So there you have it. What will this accomplish? Absolutely nothing, people smarter and more articulate than me have been arguing this point for a while to no avail. But I'm bored so I typed this anyway.
Also I hope that the nonviolent, nonsexual Travis Touchdown gets in Smash as well. But given you found Bayonetta to be a nonsexual game, maybe you just enjoy No More Heroes as the best shonen there is.