I think “revived dead characters“ AUs are, honestly, far less necessary for Undertale than they are for most other things. For example, Asriel.
Asriel is alive. When people talk about “reviving“ Asriel, it’s as nonsensical as talking about “reviving,“ say… Snowdrake’s mother would be. She’s already alive, even if it’s not in the same way she was before. It’s the same with Asriel.
His body is made of vegetation and determination, instead of dust and magic. He burrows instead of walks, he can keep things open or closed by growing vines but has no hands or feet. And…he has no soul, and doesn’t feel love.
Finding a soul for him, whether in an AU or a far-off dream of a Toby-written happy ending, could help him with that last thing. But I’m not sure just a single soul, whether it be the weak soul of a monster or the stronger soul of a human, has enough power to physically turn a flower into a full-fledged boss monster.
The Amalgamates don’t have the same forms and capacities as their components once did, but. Just being with the families that they love and whom love them, they seem pretty happy to me. If Asriel’s not-feeling-love problem got resolved… would living with the rest of “being Flowey“ really be that bad?
Undertale already is the “everyone is alive and friends” AU
Also like It took the equivalent of seven human SOULs for Flowey to become Asriel again, and you could argue that even then he still wasn’t the kid he was before (he’ll never be), he was a god taking on a boss monster form Having only six SOULs under his control didn’t give him his ability to love and feel compassion back Giving him one SOUL would make him more powerful but it wouldn’t turn him back into a boss monster and wouldn’t make him love again Only the godlike powers of seven human SOULs (or equivalent) can do that. And even then, after all he’s gone through he’ll never really go back to how he was before his death.
I agree with you that Omega Flowey’s physical form probably reflects the limits of six souls, that he couldn’t actually shape himself into a boss monster until he obtained an amount of power equivalent to seven human souls. But I don’t agree that him regaining the ability to love was a function of that power.
After all, ordinary monsters and humans in Undertale’s universe are able to feel love just fine with only one soul per person. Why would Flowey be different?
So I think, for Asriel to love again, it’s not that a certain amount of souls is needed. It’s just that, as @saveloadreset put it, you need to “prime the pump“ to get the body and soul properly connected on an emotional level.
But yea…even with body and soul fully restored, it would be unfair to assume that Asriel could go back to exactly how he was before he died. All the trauma and guilt from Chara’s death, his own death, and his rebirth as Flowey… wouldn’t magically go away, you know? Nor would “Flowey’s” bad habits and mindsets. But I think he could more safely heal and be happy if he had an opportunity to be comforted by love and supported by people who love him.