I have an idea for a new au; FrostTale (or undersnowed idk yet)
Gaster escaped his fate, frisk falls down, but Asriel never came back as a flower, but during a new run, the player decided to do a genocide run, but gaster fell into the core as frisk was fighting him which turned the core off because the energy fused with Gaster's soul and went with him into the void, and since the core keeps snowdin from freezing the monsters, they freeze and frisk's save star freezes *reference to the blue screen of death* and now frisk is forced into an unresetable timeline of lonelyness, gaster chooses to increase their neverending hell by fighting frisk in his void form, since the timeline is never going back to normal anyways
The fight ; gaster uses his hands the most and bones, with gaster blasters used after frisk starts to actually drain his hp, and all his attacks apply KR and an effect called frostbite which slows frisk and stacks 3 times, one frostbite removes 6.5% of their movement speed