You know, just to connect the RGB cartoon with the film series. They could have small parts in the movie if decide to do another “rise to fame” montage like in the previous movies. Just a thought.
I think they will go with a new thing. It has to be a BIG “welcome back”!
i dont see how a ghost from RGB wouldent be a big welcome back…a super star like sam haine or the boogie man
in gb3 would be great even if they werent the main baddy.
I think it would be cool to have some minor references. Perhaps during the sequence(s) when ghosts are running amok in the city. It would be cool to see the Boogieman in the flesh 
or perhaps my fav…
I think the cartoons tying into the movies is fine, but the movies tying into the cartoons is not a good idea. It would be an awful idea, though some of the character villians from the cartoon were based on mythological entities, so they could still appear and be done right.
But a tie in with the cartoon would be silly.
What if it isn’t a tie in, but a retelling of the same ghost?
If they just reintroduce Samhain or Boogeyman in a darker, more realistic aspect.
That would be cool.