Well that sure was an ending. Crazy how a game with crude-ass SNES graphics can make you tense up.
I'm really curious to see where Toby goes with this alternate universe and whether he'll tie it directly into the Undertale story at all. Someone suggested it's Chara who is messing with alternate universes, but I wouldn't discount Asriel's involvement either. He had that speech during the final battle of Undertale's pacifist run which was all about how he wanted to keep playing with you, and here's this elaborate alternate reality where Asriel got to grow up with his human brother, and that brother ends up having an adventure with Asriel's alter ego? Definitely strikes me as a wish-fulfillment fantasy he might have, and this could be either a timeline he somehow made with his reality-bending powers, or just something that he's imagining.
I actually kinda hope this sequel(?) doesn't get too deep into the lore and keeps the connection to the original Undertale relatively ambiguous, because I think it has enough hooks of its own to be a self-contained story. I really liked Susie's arc in this episode, and there was some intriguing side story stuff too, like Noelle and her dad.