So... I know I had an embarrassing mishap on this thread prior, but after acquainting myself more with the series and all that, read the visual novel (forget what version though) and Hollow Ataraxia, the anime adaptations, played extra (and watched ZVN's translated Gil route for CCC) Extella and Link as well, and sold my F2P soul to FGO... I'm up for the King of Knights joining the roster.
I know I saw earlier something about alternate costumes, personally I don't think she'd have anything like Lily, Mordred, or Nero as alts, I think she'd probably just have her and Salter with four separate costumes each or alternatively: normal Artoria, Salter, and then Arthur. That said, as Joker showed off the alternate doesn't need to have an equal amount of costumes as the normal costume, so it could be like: 4 Artoria, 2 Salter, 2 Arthur, or it's split up-between Artoria in her armor and then her casual wear, then you have the other two I mentioned. I'm only saying this because I doubt they'd make a design for Arthur Alter just for two slots of a Smash Bros. costume.
I'm saying Arthur because when there's usually a male and female version of a character available, Sakurai and his team usually make them the alts, I also think Salter would just be too good/easy to pass up on.
That said, I feel like they'd probably make Shirou a "background" character like how Red/Leaf are for Charizard, Ivysaur, and Squirtle. (that said this idea I'm about to propose is only if Shirou acts like that, because I can very well see the possibility where they make both him and Saber playable in the same character slot, with Shirou probably having to take some abilities EMIYA shows off in Extra, CCC, Extella, Link, and FGO to fill out his own movset.) if it's like that, I feel like the alternate costume could also affect the "master" who appears in the background, Shirou Emiya with regular Artoria alts, Ayaka Sajyou with Arthur alts, As someone who'd personally want to have the Ritsukas and Hakunos be Mii Costumes at the very least in this scenario, and be actual "characters" later on if the Fate/ series gets more reps and returns to Smash Bros with their own servants (AKA Mashu and then Nero, Tamamo, and Nameless... that said, if for example... Rin and EMIYA became an assist trophy, I'd imagine in that scenario, Nameless would be replaced by either Gilgamesh, Altera or Charlemagne, I'd like to say Gil because class diversity, but due to waifu-ism I could very well see Altera taking that spot. Then an even bigger doubt is to reveal the Berserker class servant Hakuno had in CCC if they go Gil route, most speculate this to be Enkidu, but that hasn't been confirmed so it's probably best not to do that), Salter would probably have to have Sakura Matou in the background for her alts, either normal or dark, probably dark, but... who knows.
I feel like spirits would be handled as to how Xenoblade 2's are, so... the spirit would be of the master, but their servant would be an extra opponent to fight within the battle itself, though I do feel like some exceptions would be made, like Xenoblade 2's spirits, where some servants would have their very own spirit fights, probably Gilgamesh and Sasaki Kojiro due to their abnormal situations/status in Stay Night, that or Gil gets tied to a Kirei spirit and Cu gets his own, or you tie Cu to Bazett in that scenario and Angra gets his own separate spirit.
That said, I do think all of the servants would have stage background cameos like how the Phantom Thieves do in Mementos, Yggdrasil has its own, and then how Banjo and Kazooie's friends do in Spiral Mountain alongside Gruntilda and a standard enemy.
I don't know if there's a common consensus on what the Fate/ stage would be, but I assume like how the other visual novel rep threads have on here have, think a travelling stage is definitely in the realms of possibility, in which in this case, would most likely be Fuyuki City.