Hello! As Co-Founder of
ProfessorLaytonUnion, here's the answer to your question:
Original Characters (OCs) and/or different franchise characters (i.e.
Ace Attorney, Ghost Trick, Pokemon) are acceptable as long as a canon
Professor Layton character is present!
Regardless of context, those types of deviations/fanarts would be considered as "Group Works" art, since they contain more than one character.
Thank you for your time and patience! Thanks for asking!