The thing about those big name media brands. (Undertale especially) is that as impressive as they are. They're also flash-in-the-pan moments of brilliance. No one who creates things like that expects them to explode the way they do. They often hope it will. But they're not expecting it to. And oftentimes when it does, it's for reason they weren't taking into account.
Which is what oftentimes makes their follow up installments so disappointing. Because once theirs multiple versions of it, or things that try to emulate it, it stops being unique. Nothing will ever be as good as that original. Ever.
Which is why I think it's better to instead focus on doing something that has been done before. But doing it really well, and adding your own twist. Personally, I take inspiration from the John Wick movies. Are they original? Not really. Are they fun as hell? Abso-freaking-lutely! And what's so great about that formulae is that, they can conceivably go on forever and no one is going to complain. Say what you will about the Fast and the Furious franchise, but they're on a similar boat. And until they stop being profitable, until they stop being fun for general audiences, you can bet there's gonna be more.