I can't suppress this desire... I want Kuroshitsuji to be a movie so bad!! I would love for English film makers to give it a go. If they're as true to it as they were to Harry Potter, it has no chance of failure. (Of course, they were only so accurate to HP because Rowling forced them to give her consent about all their decisions)
BLACK BUTLER SEASON 1 SPOILERS AHEAD:
I think it would be two movies... The first would be all about introducing the characters and the hidden plot behind Ciel and Sebastian's contract. The MAIN plot however, which would probably be introduced a 3rd of the way into the movie, will involve the Jack the Ripper story.
Therefore Grell would be the main villain who gets confronted at the end, and of course it would be a surprise since throughout the movie he's the overly dramatic suicidal butler. Even though you're transitioning from cartoon to real people, I think Grell and certain other characters can still have their quirky antics. I mean look at Zeus from Tron. That guy was insane and hilarious! Every single character is ridiculously interesting.
Also as another food for thought, look at this guy I found who looks just like Ciel Phantomhive: http://www.halloweenplayground.com/spartacus-pi-3167.html?osCsid=a39083b5fb887e5d5de668a941565167 He's obviously way too old to be considered, but I just felt like sharing it. Does anyone else think a live-action movie could work?