Speaking of Gaster, I laughed at how some people for some reason believe that npc is the key for the supposed "real true ending", in which allegedly you would fight the Fallen Child and somehow save Asriel from its terrible fate, because they don't like enough the Good ending or something. It turns out the game's criticism about OCD completionists never being happy was correct.
In fact, if Toby wants to be cruel, he could toy with that idea and add the ability for the player to "save" Asriel, in exchange of paying the ultimate price: giving up his soul and everything he worked for. Ingame that would translate into having your save data erased and your character's name permanently rejected by the game's register (in other words, doing something similar to what they did in NIER). The following butthurt would be hilarious.