Gaster <> River person swap concept for my own personal undertale AU I quite love to draw now and again. A big part of the AU is how the Monsters won the war, and thus were free to take the rich underground for themselves.
Ol' Rivers the Explorer, is one of the foundational members of the Underground in this AU, as it was their efforts that allowed the untapped riches of the ICEWALL to become inhabitable, along with most the underground. Tragically, they disappeared along with their crew of explorers in a dangerous trek through the most untamed part of the Underground; the Untamed Frontier, or just "The Fringe," trapped in their own obsession.
This concept is really old, but since it's kind of the thing to give your cool undertale aus a nice name, I name this Underswap AU of mine "Phoenix Pitied" while the Deltarune equivalent is "Expedition Phi", which ngl, I worked backward from there haha. Still works though, because the Underground is gonna be depicted as a crumbling empire, a phoenix so to say, that will rise from the ashes.
Anyway, that's all, good night!