Groundsy the Groundskeeper will be the Big Bad
We know there are multiple evil teachers at the school, maybe Groundsy is one of them. Also, it'd be hilarious. Higglemas turned his brother into a dog and is secretly running the school
Higglemas called his dog Hero, which is very close to Heiro. The Heironymous we see is just a construct/illusion, and the real one was polymorphed into a dog. Higglemas, tired of being overshadowed by his brother, or possibly thinking he could run the school better than his brother, turned him into a dog and has been secretly running the school from the shadows. Perhaps this is why he never leaves his office, he needs to be there to keep the spells going/he doesn't want anyone to know he is using those spells. He cares about his brother enough that he does no want him to die, hence why he has Firbolg check on the dog to make sure it's healthy.- Confirmed, in a sense. Higglemas turned his brother into a dog, not out of jealousy but to protect him from a devil prince that was attempting to kill him.
A future arc will have the gang meeting their future selves.
Either Travis will be playing their future selves, or everyone will be Acting for Two. Either way, it will be hilarious.