Although I like the thought of Koopa Kid being a major villain, something strikes me as cliche, as Koopa Kid exploits the usual diversionary tactic people use in movies, and we get this father-son thing which I think feels a bit forced anyway. I want to be minimal on the original character thing and rather develop the ones we have right now, especially Mario and Luigi.
I'd like during the movie, there's a lot of bro-to-bro interaction between Mario and Luigi. It should be emphasized that they're not just brothers but
twin brothers. Yes, being a twin (who's identical), that makes a whole world of a difference. We don't have a lot of stories starring twin brothers
(much less twin sisters)