If Johnny and Mavis ever end up having another baby, their second child will end up being a girl who they named "Martha," after Mavis's mother.
The demonic bats were once human.
Vampires in fiction often have servants. The bats we see are humans Vlad has turned into servile vampires, by stealing their soul.Stealing the soul of a human removes any part of humanity from them, causing them to become grotesque over time and more animalistic and bat-like.This means, indirectly, human-turned-vampires are the real monsters.
Mavis will turn out to be pregnant a second time at the end of the sequel
Although she and Jonathan obviously love Dennis very much, I imagine that they'd probably want to have more kids, and that Dracula would want more grandkids—even though he also obviously loves Dennis very much.
Johnny will have to disguise himself as a monster again
Vlad apparently is a very old school vampire, which means he probably doesn't like humans similar to how Drac was in the first film (or even worse). So they might try to hide the fact that his grand-daughter ended up with one.
Because, why not?
The climax will have Dennis flying.
It might involve him saving himself
or one of the other characters from certain doom.
Dennis will not grow/exhibit fangs or any other vampire trait
Just a personal theory of mine. It would serve as a next step for Dracula to accept his grandson is more human and that Mavis needs her own space/her own decisions.
- Jossed. He gets his fangs in the climax.
Unlike some portrayals of vampires in media, these vampires do have a heartbeat.
Since Dennis' lack of fangs and obvious vampire powers is the main focus on determining if he was more human or vampire like, that implies that some of the other obvious differences between humans and vampires aren't in place. A lack or presence of a heartbeat in the kid would seem like a more noticeable difference over the years. Especially when he was small and someone was holding him or rocking him. If both humans and vampires in this universe have a heartbeat, then it wouldn't be an obvious clue.
Of course, that still leaves the question of whether or not anyone noticed if Dennis had a reflection or could be photographed at some point during the five years...
- It could also be due to the differences between Dhampyrs and vampires. Maybes dhampyrs have a heartbeat and can be photographed. Dennis apparently isn't harmed by sunlight, unlike his mother and grandfather, and he eats regular human food, while Dracula and Mavis seem to eat what bats eat. Maybe Dennis gets all the traits of a human body (ex. human skin, heart and stomach), plus vampire powers, and no vampire weaknesses.
These vampires are also not immortal.
They are certainly long lived, but they're not immortal. Dracula is visibly middle aged and Vlad is an old man. We know Dracula started life as a vampire from what Vlad said, which means that wasn't the age that Dracula was turned since he never was turned. Also, Vlad's vampire teeth have fallen out and had to be replaced with dentures.
Female vampire hunters can make monsters zing for them to make them easier to kill.
Which would explain why in the trailer for hotel Transylvania 3,We see Dracula zing, but not that woman. She is not interested in him in any way obviously, so she did not zing.