KOS-MOS would be great but I've been wanting her for a really long time and she's never happened yet so I'm not sure what her chances actually are. I name my retainer in FF14 Kirschwasser after the ones in Xenosaga. KOS-MOS is really cool but it's hard to imagine why Nintendo would pick her.
I don't know how long you've been wanting her for but circumstances are very different now from before, especially during the Switch era.
Nowadays Monolith has become Nintendo's biggest subsidiary, and bigger than most of Nintendo's affiliates, including stuff like Intelligent Systems which has existed for a lot longer.
And the reason they've been able to grow so much is precisely how successful they have been. Obviously with Xenoblade they are supplying Nintendo with a more traditional JRPG series, something Nintendo has been struggling with since the N64 era, but there's more to it. Their most intimate codevelopment with Nintendo, Breath of the Wild, turned Zelda from a sub 8 million seller into a 20+ million seller. And while not to the same extent as BotW (where half the development team was Monolith according to interviews with Miyamoto and Aonuma)
they still have a hand in other major Switch multi-million releases.
Probably the main reason why I'm confident in a Monolith character happening is the stark contrast between how Monolith was during Smash 4 and how it is now. Back then they weren't half as big, they hadn't even had a million seller with Nintendo yet, and despite that they got Shulk in the game, a stage with a boss, an assist trophy and even a significant piece of content from one of their pre-Nintendo series:
a remix of Valedictory Elegy from Baten Kaitos. I know it doesn't sound that major but imagine how such a thing even happened, either Monolith themselves or somebody at Nintendo thought pre-Nintendo Monolith content was worth putting some effort into
and pushing for it in Smash (also personal ranting: that remix is much better than the one Xenoblade got, which is just a bunch of tracks forcefully stapled together and barely sound any different from the original).
However what does the much more successful Monolith get in Ultimate years later? Absolutely no significant content... obviously no new character, stage, musical arrangements nor items, you know, the things that actually require effort from the Smash team. It got a couple mii costumes, but they are obviously just scaled down versions of XB2 models when you look at them closely. It got a few, very few, music tracks but none of them are remixed (
also they are still bugged years later, what the ****). The CLOSEST thing to new significant content they got was Mythra's redesign... which was handled by Monolith themselves.
The dichotomy just seems so strange to me you know? It would be one thing if Monolith was still their smaller selves from the early 10s, then it would be far more believable to me that they could get completely skipped in a smash game. But now? Even in the most pessimistic and cynical image of Nintendo it's hard for me to imagine they would be like "yeah, these guys and the stuff they make are no good, they don't make any money. Let's not bother" and for this mentality to endure for the whole 6 years Ultimate's development cycle is going to last.
And if I'm wrong? Well in that case I get to save some money. I still have to recover from the critical hit Kosmos dealt to my wallet...