A Gravity Falls game would be neat but, in that particular case, I'd advocate for a different art style. The characters look too cutsy and young in the cartoon. A good compromise is what the comic
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did: you can see a semblance of the cartoon style in the art - however, it was 'matured' to accommodate the fact that the characters are now all 18+.
Danny Phantom, OTOH, already had a 'mature' enough art style to make the characters look 18is - as the other IPs you've mentioned.