Bo's parents/family will appear in a future season and will have episode relevant roles
Bo is the only child whose parents did not appear in the Parent's Day episode in season 3. They're most likely saving their appearances for a future episode where they have a prominent role.- Confirmed and Jossed. They had a non-speaking cameo in the Christmas Special
Aliens will be heavily used in Season 4
In "The Abysmal Altercation of the Abominable Altitooth", there were two scenes that featured an alien trying to communicate with Earth in order to deliver an urgent message. The aliens and the content of the message will be relevant in season 4.- Elaborating on this, given how George referred to the alien's attempt to communicate as "boring", the aliens that show up will want revenge for the boys' disrespect.
- Confirmed, with the series arc revolving around the conflict between Littlites and Biglyans, the latter of which kidnap Melvin.
Erica will eventually get Humble Pie by running into something she can't predict/plan for.
Possibly the aliens from the above WMG. And in addition to Erica being humbled, George and Harold will question some of the things she's told them, including possibly Krupp being born evil. There will be a crossover with Dog Man, which will then spin off into its’ own series
Considering that Dog Man is already a creation of George and Harold in-universe, I can see this happening. Mr. Meaner’s twin sister Misty is Transgender.
It would explain her Vocal Dissonance and why the only physical difference between her and her brother is her hairstyle. Mr. Krupp/Captain Underpants will become a Monster of the Week in an episode.
George and Harold will have to call upon Sergeant Boxers and Plungerina and also use Melvin's inventions to all come together as a team to de-monsterfy him. Mr. Krupp will keep dancing for the rest of his life as principal.
Jossed.