I think it's interesting watching Johnny play a dark Character

I believe so , because Johnny was shy and kinda dorky as Edward Scissorhands and as Bill Blake in Dead Man and he was really reserved as Gilbert Grape and really inconventional as Sam in Benny and Joon! Newt was all the above, was kinda insecure and indecisive and clumsy, but in the same time special and unique! Johnny has proved himself in that type of roles and he even could take them one step further, thus my opinion!!!!!!!!!!!!!

Of course it's interesting to see him in the dark character's shoes, but, as it was ,Newt had a bigger part although Grindelwald''s name was in the title! I'd die to see Johnny be the protagonist again!!!!!!!!!!!!!!!!!!!
He is just the one you fall in love with and forever will be!!!!!!!!!!!!!!!!!!!!!!!!! With the existing cast I think I'm falling for Grindelwald!!!!!!!!!

I think out of all the characters, Grindelwald was the best role for him. See, all the movies you mentioned "Benny and Joon" "Edward Scissorhands" and others were made more than 20 years ago. Johnny has grown up. He is not a boy anymore, he's a man. Newt is a young guy, if "Fantastic Beasts" was made any time between 1990 and 2005 then Newts part would definitely be perfect for Johnny, but now he's too mature. I would even just say he's too old. He needs mature characters now. He definitely was perfect as Grindelwald and after all, we all love that bad boy vibe Johnny has always had!