Ranni in lore is a very capable magician, though we don't see her fight in-game. The closest would be that under specific circumstances in her quest line she'll one-shot you and kill you, and the Renalla fight is entirely an illusion created by Ranni, she's basically a magical construct Ranni makes. Because of that I think it would be fairly easy to interpret a moveset for her, and you could fill out her moveset with other Glintstone sorceries and it wouldn't be weird at all.
That said I do think the Tarnished would be the way to go for an Elden Ring character. I can see the appeal in doing Malenia, but the core gameplay of the Soulsborne games is one of their most iconic elements and is extremely influential. The Tarnished conceptually has like 20 different movesets that could all not share a single move and all would make sense, but just the core element of "stamina-based dodgerolling patience and punish fighter" would feel distinctly FromSoft and already set the Tarnished apart. If you're including a Soulsborne character, I think it would be sort of a shame to not represent the gameplay that makes the games so memorable, and it lends itself to Smash's general design philosophy best.
Ah, I see. I do agree on that point - Sakurai generally tries to represent a series' core gameplay, and doesn't worry too much about blank-slate avatars, as evidenced by the likes of

(And others probably, but I can't rattle them off from the top of my head). Hell, he even managed to get Hero, despite DQ's long-standing tradition of not letting them in crossovers - The series' usual crossover rep, Slime, was only his Plan B if Hero wasn't allowed.
That said, there does seem to be a couple exceptions to the "player character first" mentality - Namely, Sans being Undertale's sole representation and being met with tons of hype, because he's
Sans and is just that beloved. Sans' popularity is a whole new level of virality and I don't think there's any Elden Ring characters that are
that popular, but I have heard that some of the other characters I mentioned are still extremely popular in their own right, which is why I mentioned them.
That said, unless the likes of Ranni or Malenia or whoever else are really are
that overpoweringly popular, I am inclined to agree with you - Soulslikes have a very distinctive sort of gameplay that would set the Tarnished apart from the rest of the roster
Something I mentioned before is that I'd like for the Tarnished's moveset to include the Dark Moon Greatsword. A version of that sword appears in almost every Fromsoft game, so I think it'd be neat if it was included in some way in Smash. It's also obtained in Ranni's questline, which last time I checked, leads into the most popular ending according to Steam. Give the Tarnished some Cold sorceries and they could have an interesting gimmick of building up Frostbite in their opponents, causing massive damage if it's allowed to build up.
Yeah, I've heard of that - Usually it's the Moonlight Greatsword, right? I don't know if there's a "default" weapon always used in the marketing, but if it's that significant of a FromSoft staple, the DMG sounds pretty close to that. If it can't fit into a moveset, I could at least see it as an item, though something that important sounds like a better fit for a fighter's moveset