Gaster will be brought back to life.
We know that Sans and Kurt are trying to bring pieces of Gaster together, sooo...- Confirmed!
Frisk will eventually participate in the Book 7 event of Harry Potter.
Seeing as this lore is similiar to Harry Potter, it is eventual that Frisk will face off in the war for Hogwarts. Chara and Asriel will perform another Fusion Dance to try and take down Voldemort.
And it/they will fail. Sans will die in book 5.
Seeing as the Mt. Ebott monsters won't be willing to join Voldemort, Voldemort will try to kill them. Sans, will most likely face off against said dark lord, and will fail.- If not, he'll give Voldy one hell of a bad time.
Voldemort's Horcruxes will actually work against him.
To make his Horcruxes, Voldemort splits his soul. Monster magic targets the soul, not the body. As a result, it might be that Voldemort is weak to monster magic due to his soul being far weaker than normal.- Maybe, but he will also have an HP boost from having high LV.
Asriel will have a scathing article from Rita written about him, and in Book 5, Umbridge will nearly or actually expel him, or install a 'security detail' around him including Draco.
Extremely likely due to him being a monster/magical creature attending Hogwarts and using a wand; Rita will jump on it for a sensational Prophet article and Umbridge is just like that.