Finally ! Since I didn’t have the time (or courage) to do all the story, here an illustration that sums up the rest of it.
A rapid summary of my idea : Frisk condition got worse and she is plonged in an artificial coma. Since he can’t count on the humans, Sans takes matters in his own hands. As the saying goes, “you are never better served than by yourself”. He goes to the True Lab with Frisk. And here begins Sans’s mad race to find a cure or anything to save Frisk. While her body deteriorates, his mental state also. He takes her soul everywhere with him, having discussion with it. But is it an illusion or Frisk can actually communicate with him ?
Here you are, a sad story. So, does it exist already ? Does it have a name ?