Sonic Headcanons Based Solely on the movie:
Maddie's not straight. Bi at LEAST. Don't know why but she is.
If Robotnik is mentioned or talked about by ANYONE in the Sonic family, he's called Eggman, don't @ me on this
After Robotnik gets back, things are relatively normal
Robotnik tries to get Sonic several times and fails, but its no where near as intense or dangerous as the first time
Robotnik is not out for blood. He just wants to test somethings with Sonic's power. But he never actually explained what he wanted with Sonic, so they assume he wants him dead
Robotnik and Sonic argue like little kids ("Am not!" "Are to!")
Stone and Maddie meet up for coffee without Tom or Robotnik knowing
Sonic views Ivo and Stone as his weird uncles after a while
At one point, Robotnik has Sonic in his clutches. He tells Stone to let him escape.
They're all still enemies but Robotnik and Stone are invited to things like Thanksgiving and "its stupid to turn down an opportunity to spy on them, Stone, and Maddie makes excellent turkey"
They all know Robotnik won't hurt them but he threatens them anyway
Maddie picks on Tom for letting a minor into a bar and then proceeding to start a bar fight
Sonic and co try to get Robotnik and Stone together
Tom and Sonic tease Robotnik A LOT about his obvious feelings for Stone until he admits it (which he won't until he absolutely has to)
Maddie (who has the braincell of the sonic family) legitimately helps Stone figure things out
Robotnik does not legally exist and he uses it as an excuse not to do things ("you can't do that" "I Do Not Exist")
Robotnik has absolutely dragged Stone into one of his dance sessions
Sometimes, Tom and Maddie can't pick Sonic up from school so you might just see this giant, egg-shaped robot thing pick Sonic up and take him home (or sometimes a guy in a suit who Sonic will address as 'Rock Man' when he gets in the shiny black car)
Ivo will deny that its him if asked but tbh everyone knows
Even though they should... nobody told the government that Robotnik it back.
Stone babysits for Sonic "without Robotnik knowing" (he knows but ignores it)
I just really love this concept-