I would advise staying away from Superboy, Wonder Girl, Kid Flash, and Aqualad as the center of the story since they are so close to their adult counterparts in terms of their abilities and typical roles within the team. Not saying that they should be completely absent from a live action Teen Titans movie series, but for the starting line-up, one close to the cartoon would be best (Dick Grayson Robin, Starfire, Raven, Cyborg, and Beast Boy). Robin, of the sidekicks, offers the biggest difference between himself and his mentor as opposed to the earlier mentioned heroes. Then Starfire brings the sci-fi, Raven the fantasy, Cyborg the tech, and Beast Boy the comic relief/lovable animal guy. Then of course they could build off of recognition of the TV show and all that.
The tone of the movie doesn't have to be completely lighthearted. There is a big potential for a dark story especially if the story focuses on Raven or even Robin if they decide to handle his character in such a way. There'd definitely be a good deal of humor through Starfire's outlandish nature and hopefully the friendship between Cyborg and BB like in the cartoon. However I think this could pull a solid PG-13 rated action movie with a good plot and sense of danger.
Also, I would literally go watch this movie every weekend if they including the original Puffy Ami Yumi theme song in it somehow.