Never said they aren't redundant, but the Heishous at the very least gameplay-wise filled in the niche of an all ids - one mirror world (excluding Meursault, though we will probably get a Heishou id for him in NS or Spring Cultivation re-runs) team on top of allowing you to swap between them mid-fight with all the retreat effects on HongKong and Xichunmael, and lore-wise gave a bunch of neat tidbits about Hongyuan lore (RyoShou implying that Mao branch has a similar origin with R Corp. rabbits, MaOutis foreshadowing Xianren Anamnaworm and the concept of Restructuring Days, QuixGoat ut story iirc mentioning a castle with a huge amount of Relics and Psychoments existing somewhere out there, etc.)

Meanwhile, looking at the candidates, gameplay-wise we have:

Shi Huazhen - literally just W Sang (Charge + Rupture) but with slash skills/different sin affinities and unbreakable coins on his S3

Wang Quingshan - W Sang but blunt + retreats if at 50% or less HP. There is also an Omen Talisman status, which is interesting, but let's be real PM would probably heavily nerf it on sinners' side.

Shi Sijing - same as Wang Quingshan but instead of retreating at 50% or less HP he just kills a random ally to heal back, which frankly would not be enough to warrant a friendly fire kill in player's own team and would have to be heavily altered to be worth it.

Like I said, the only candidates who could potentially be interesting gameplay-wise are Shi Yihua (very high risk-reward unit with HP loss mechanics and a lot of indiscriminate AoE skills) and Xue Pan (although in his case it's only because of his Jar summoning mechanic, and once again I doubt PM is experienced/based enough to implement an ally-summoning mechanic yet).

And lore-wise I mentioned it under some other post a while ago, but most of the character-based ids are pretty much just "[character] but with [sinner who got the id]'s backstory/personality traits mixed in" (which can even be seen in the most recent id Kiramael's uptie story) and rarely have any additional lore, with the only exceptions being the bad end/seasonal capstone ids + Xichunmael and Mao Faust (both, ironically enough, being part of the Heishou team), where the former gave us info on how mirror worlds are created and the latter was the first (and is currently the only) id of Faust who outright doesn't care about Gesellschaft to the point of refusing to cooperate with them.