I know it's not mainline, but Undertale Yellow just released and its actually really good. I'm not done with it, but it follows one of the previous human children who fell down, and manages to be its own story that explores different parts of the underground and different people. Like you meet Toriel early on, and then through mischance fall down a second time into a different part of the Ruins and come across a PTSD-stricken vampire(?) called Dalv who you follow through the ruins to his home, fight, and have a chance to convince of your peaceful intentions and befriend. He's a sweet guy, or so it seems, wants to become a children's book creator.
Similarly, the closest I've seen to a major character encounter is that I got to see the Ice Wolf.
(Also there are new monsters to fight or in this case not fight, tons of puns and silly characters, etc, etc.)
It definitely feels like an UNDERTALE game, but it knows that to do it, it can't just repeat the plot beats of Undertale, complete with meeting Sans (at least not yet, maybe he'll make a cameo?) and all the crew. Especially since despite the fact that it's kinda a doomed game in the sense that we know it ends badly, you can in fact befriend people thus far and win without fighting.
Meanwhile it's quite clear that Undine and Papyrus haven't joined the friend rodeo before, so of course they wouldn't meet Clover the 'gun' slinger, who apparently went to the Mountain to try to find a missing person (who is almost certainly dead
)
Oh also, your new good friend Flowey can help you SAVE your game, and give you all sorts of advice about how annoying the monsters are and that maybe you should do something about it.
gamejolt.com
(From what I can tell, a full playthrough I saw on Youtube but didn't watch because I'm playing it for myself clocked in at around seven hours of content minimum? So, like, there's apparently a lot there, even if you don't dilly-dally.)