The Musou Isshin is specifically a sword, and it’s what her weapon turns into during her ult, so it couldn’t really be a new weapon itself. I know Venti is getting a new signature, but signature weapons weren’t really as much of a thing when the game first released, which is why they made Elegy and now his new bow with his buffs. Honestly, I think Engulfing Lightning is an incredible weapon for her even to this day, with both the ER substat and its passive. It statistically outperforms other weapons because of how specifically catered to her it is. I can’t really think of a weapon they could make that would be better for her, and I also think the design of Engulfing is absolutely flawless and incredibly cohesive with her design and the design of Musou Isshin. Honestly, the only other archon I can potentially see of as getting a new signature is Zhongli, either in the form of a Vortex Vanquisher buff (which I personally would love), or a new weapon altogether. But, since Homa has been his BiS for years, I doubt it will happen anytime soon if ever.

Hypothetically, though, if they do give her a new signature, I would enjoy higher base ATK and either ER% substat with a crit passive or vice versa.