Im gonna say Artoria, or anyone from fate, biggest issue is that Sony is very closely tied to Fate recently. Not sure of its Just Grand Order or the whole franchize. If its the later, one has to wonder if Sony wants to work with nintendo at all.
Considering the next Fate/ game is going to be Switch exclusive, and its possible the Extra and CCC remakes, as well as even the Tsukihimie remake may release exclusively on switch in terms of consoles. They literally have no control over the series outside of what Aniplex does.
That said actually, Aniplex are owned by Sony /Music/ Entertainment, not Sony Interactive Entertainment which control the Playstation stuff. That said what Aniplex does is essentially only helping out with the anime adaptations though the exacts evade me, I'm quite sure its financial. Besides that it's just FGO (and its figure trading game spin-off) of where once again it's probably just an financial thing alongside advertisement.
Otherwise everything is owned by NOTES/Type-Moon.
We've also had this talk before, but to add on to whats already been said: several companies, including those in Smash already, essentially ignore the eroge past of Fate/. If they didn't I'm sure we wouldn't have had Capcom publishing Fate/ Unlimited Codes, SEGA being the ones responsible behind FGO Arcade, and going outside of Fate/: SEGA publishing the Melty Blood series as well.
Point is: Fate/'s lewd past isn't exactly "remembered"/isn't cared about by the overall game development community. any of Type-Moon's games that had that content are also applicable to this. As others have also said they've basically erased those moments from existence all together with the remakes/re-releases of Stay Night under Realta Nua and Hollow Ataraxia's Vita/PSP port alongside other material. Of course though they're gonna do cheeky references to such acts but honestly: a lot of media, even stuff for kids, has dirty jokes referring to lewd intercourse.
But yeah, Assassin Jack... weird flex but
w h a t e v e r. The only solace most have to try to make it less weird is technically if we count "birthdays" past death that which makes up Assassin Jack legal by now... but that's also an sick excuse for certain types of people. That said I prefer Zerker Jack anyways... god I wish the writer of Strange Fake would let them be used in FGO by now with the snails pace he's going to finish it but... it takes time to finish stuff like that and I'm more paintent than most so I'll continue to wait.
Edit:
Also when it comes to VAs, Nintendo is fine hiring them as long as they're: not in a union or if they're willing to not be credited as themselves (or whatever work around most union VAs find to voice in Nintendo projects) but are forced by the contract with the company of the character is owned (in this case, Cloud's old VA, unaware for his new one) to always be credited. So as long as the VAs behind X Fate/ characters don't fit those criteria or are willing to do whatever loophole other Union VAs do they should have no problem reprising their roles for Smash (this goes for English and Japanese, and any other languages I suppose).
When it comes to animation inspiration... there should no absolute no trouble here at all since unless for some reason they're directly ripping it from another game (which is a no-no anyways) or are using pre-existing clips from the actual anime itself for something say like the trailer... should be fine.
That said, not gonna lie... a Fate/ Newcomer trailer being done by UFOtable would be awesome.
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