So UT: After Hours follows my take on UT Hardmode's Genocide route

prologue 1 - Sans fight:
You'd encounter sans around the end of Snowdin after killing papyrus. He'd be standing where papyrus normally does, and start to converse with you. he'd tell you the reports he's been getting have been abnormal, as if something had changed. He'd give you one last chance to reset, as you haven't fully committed yet, but the player refuses. Sans' fight would be shorter, but more intense and more tricks. By the normal phase 2, sans would have already exhausted himself, and the attack would get significantly easier until he passes out. Sans also uses his fabled "special attack" which turns out to be a janky machine that he had been working on in the back of his house. When activated, it would Be able to reverse time (the attack) and that would also invert your controls Sans is then struck and killed, and the player continues onto Waterfall

prologue 2 - Grillby's:
you'd be met with a cutscene of grillby's with the Normally surviving people inside of it. They'd be huddled around the jukebox listening to MTT News, where MTT tallied up the confirmed deaths and injuries. MTT would then list Sans and Papyrus, and Grillby becomes furious, but quickly calms himself. He tells everyone he's closing up shop for the day, and that they could stay there if they needed shelter. The customers keep prying Grillby's, but he won't say where he's going

Extras - Possibly a papyrus, Undyne, or MTT fight; Maybe others

Phase 1 - Grillby's stands alone in the last corridor, determined to make your life hell for what you did to his neighbors, friends, and family. he would use fire based attacks, and his mood would heavily effect his stats. Being a fire monster, Grillby doesn't take damage in the normal sense, and more or less through magical energy, that keep his flames high.

Phase 2 - Grillby's loyal customers appear! (inspired by monster mash) They would help him fight the human, and also make it harder to manipulate grillby. They would fight in turns, solos, and All together, depending on who needs to rest (keep in mind these are just regular old monsters)