Oh boy, I'm schizoing again!
Personally, I'm all in on UT Asriel/Flowey somehow being Ralsei rather than the headband theory, which feels rife for a fakeout.
The creation of a Dark World would've allowed for Flowey to actually feel emotions without having a [Heart Shaped Object] of his own, and Sans is nearly walking proof that travel between the worlds is somehow possible.
Also, if Ralsei is just a simple physical object, then he can
absolutely come to the festival, sure he might not be able to talk, but he'd still be able to experience it in some way, right? Tenna experienced the events of the Dreemurr household, as did Ramb, and all that other stuff.
Unless there was something that made him really
really not want to leave the Dark World, like say, turning into an emotionless psychopath if he were to leave it.
We know that Dark World Experiments were done in the world of Undertale too now, thanks to finally understanding what Entry Number 17 meant.
Lets not forget that Asriel did turn into the physical manifestation of the Deltarune in his final form.
Kinda weird how his design incorporates 4 triangles/hearts though...
Almost like...
Strange.