Story: After the 10th genocide, Sans decides to fight you on the bridge after Papyrus leaves. He dies. Papyrus sees everything, being curious as to why the human took so long to arrive, and runs to Undyne explaining what happened. She hesitantly promotes him to royal guardsmen. They will collaborate to stop you. The encounter will be on the bridge (not sure what it is, but I think it is a bridge) before encountering Mad Dummy/Glad Dummy.

Reminder: This is only a concept. I want the internet to expand on it and maybe create their own game. Who knows? As long as you credit me, I'm fine with it.