I know this sounds strange but after playing Undertale Yellow, and seeing the discussion about making prequel fangames taking place before Undertale Yellow, It all feels kind of....Pointless? From a storytelling perspective that is.
Mostly because they would all have the same the same endings/concepts that Undertale Yellow did: You die in the Underground in order for the story to follow the "canon" timeline via neutral/pacifist routes or you deviate from the "canon" timeline through the genocide route. And at that point, the game is just Undertale Yellow but with different monsters and battle mechanics.
Or maybe I'm just overthinking things, because after all it is a noncanon fangame.