It depends whether the goal is to capture more of the "look" of the charas or the "feel" of the storyline. Highly stylized charas such as Luffy, Yugi, and Goku will never find perfect real-world counterparts and anyone heading to a live-action film expecting such will probly be disappointed. The "feel" of those and other manga creations howev I think could be captured given the right staff (director, writer, producer, etc). It is important to remember that while many manga/anime have worldwide fans the tastes of each country are a little different, so a movie can be a hit in some places but completely miss the mark elsewhere. A recent ex: DB Evolution performed much better overseas than it did in US. Does that mean Asian fans saw a different/better movie? No. But were they expecting/wanting a different kind of movie? Possibly.
I personally wld like a live-action version of Karin aka Chibi Vampire, having an interest in both romantic comedy and horror. (I'm weird like that...) Just thinking of a real-world Kenta has me almost in a nosebleed myself! Yes, I know he's supposed to have a scary face. I never saw it like that <__<