I got a Undertale - Steven Universe au
So, the humans (excluding Greg) are humans and the gems (including Steven) are the monsters. Fusions are also monsters and are their own being. I call this au "Underverse" or "Undergem" I don't know which title sounds better.
Frisk - Connie
Chara - Lars
Flowey - Aquamarine
Toriel - Rose Quartz
Napstablook - Opal
Sans - Amethyst
Papyrus - Pearl
Nice Cream Guy - Garnet
Annoying Dog - Lion
Grillby - Rhodinite
Gaster - Pink Diamond
Monster Kid - Padparadscha Sapphire..?
Undyne - Lapis
Mad Dummy - Sugilite
Temmie - Centipeedle
River person - Blue Diamond
Alphys - Peridot
Mettaton - Sardonyx
Guard 1 and Guard 2 - Topaz and Topaz
Muffet - Alexandrite
Burgerpants - Jasper
Bratty and Catty - Ruby and Sapphire
Asgore - Greg
Asriel - Steven
Asriel Dreemurr (evil version) - Stars/Larven/insert Lars and Steven fusion name here.