It really would be a design oriented AU
But I've put so much thought into this, that I kinda want to ramble it out.
I don't really have a name for this undertale AU, but here are some thoughts.
-The Monsters used to live on the surface, but of course in ancient Egypt times. Until The seven High Priest summoned an enormous sand storm, and buried their entire city under the ground, pyramids and all.
-Papyrus would be a guardsman of course, and he loves riddles. Puzzles would be more like riddles (Like a sphinx, memo, add a Sphinx somewhere in there) The best part of this is that Papyrus's name is a pun, and Sans will never let him live it down
(And if Papyrus doesn't think his name is a pun he surely is in deNILE!)
-Papyrus is a water reed, that the Egyptians made into paper (Even the word Paper comes from Papyrus, think Papyr= Paper)
-The areas would be varied, from Marketplaces, to the pyramids themselves, perhaps an oasis too. Maybe the Pyramid would be the Core, like the castle.