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.
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.
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.