It's just a mod for chapter 1. Here's the link https://gamejolt.com/games/ribbitmod/671888
I haven't played 3 and 4 either. I kinda lost interest in Deltarune. It just doesn't vibe with me anymore.
For any other game, I'd agree, but Ribbit was just SPECTACULAR. It has depth without being depressing and is just bizarre as hell. Would highly recommend checking it if you haven't.
I don't really know if it's fair to count mods in. Especially if they add to the game instead of changing them completely (e.g. Eclipse) because despite being very good, it'd make sense that they are better than the originals as they are the...
I'm not the biggest Kirby fan so I'm not sure if I'll get it, but the game looks insane features and content wise. If you liked Kirby Air Ride, then this looks to be that just a million times better.
Super Mario & The Rainbow Stars seems to be shaping up that way. Psycho Waluigi gets a lot of praise there too.
I guess stuff like Paper Mario: The Origami King Refolded and Super Mario Eclipse would just be better than their original games too...
My favourite ones that I can think of right off the bat, are
WL4 Parallel World
Ribbit the Deltarune mod
Earthbound Halloween Hack
And a lot of the Pizza Tower fan levels made with the cyop mod are really good too. A couple of them genuinely...