Dive into your creative stream
I personally believe that if enough pacifist routes, without a single neutral or genocide route occurring, Gaster, Chara and Asriel can truly be saved.
I also Headcanon that when a monster Soulbond’s with a human, the human’s lifetime is drastically increased, and their aging slowed to about monster speed.
I will add more later, these I just needed to write before I forget.