Basically, you know how Earthbound Halloween Hack takes place after the party loses the Giygas fight, with the new protagonist being from an entirely different game? Yeah, I'm doing a similar concept with this. The Batter, for unknown reasons, has appeared in the Deltarune universe midway through his own adventure. He believes that the soul controlling him is the same as the player controlling him in OFF, and that he's still on his mission to "cleanse the specters" (or, in other words, kill everything in his path, this time with darkners).

Unbeknownst to him, the former protagonists of Deltarune were killed. The Titan they fought was slain, with the power of Kris' soul being enough to destroy it, but only in the process of the party dying. The soul finds a new host in the form of The Batter, with Susie and Ralsei being in a more conceptual/non-physical state. Only when strong memories are unlocked for them do they become more physical and can join the party, though them being alive is debatable. In this state, they are no longer people with personalities, and have been reduced to tools, "party members," add-ons.

The three acts of the story would take place in reverse order of the chapters, starting with ch4, then ch3, and finally ch2. I'm thinking the first boss would be the Old Man, who feels guilty for having been unable to save the party. He doesn't really put up much of a fight or try to be some kind of badass, but his personality does still shine through a bit. He'd probably crack a joke at the Batter asking if he's a specter. In the second act, what's left of TV World has been hijacked by Eram, who is the boss at the end. Then, the final act takes place in a mostly frozen and dead version of Cyber City. The last thing there, standing in the way between the Batter and cleansing the dark world, is, well... Swatch.

Okay hear me out on this one.

The point of making Swatch the final boss is to give him more importance to the story like how the original halloween hack did with Dr. Andonuts. It's for some character contrast, cause Swatch's whole thing is color and The Batter leaves everything in his wake black and white. Swatch will probably appear earlier in the story as just a shopkeeper, doing his job but having some suspicions about the Batter, only really revealing his intent to stop the Batter at the very end. Also, Swatch was used by Mettaton to create his concept drawing of the body he wants, since he's basically an art program like MS Paint. Swatch is a tool of creativity and creation, while the Batter is a tool for causing destruction and making everything the same. That's my idea behind that choice at least.

Also I just really like Swatch and think he has a lot of unexplored potential as a character.

I mostly think Swatch would remain stoic in his encounter. Even if he's very obviously pissed, he does a better job at keeping it contained than, say, EBHH's Dr. Andonuts (also I don't wanna write him saying a certain word, you know what I'm referring to).