I keep thinking about doing Power Rangers with a more mature sensibility. I think the conceit of the show/movie would have to be totally different; the original was made to sell toys, and so has every version since then. It was even more shameless then most 80's cartoon fare.
So, if it were up to me to modernize it, I would want to make it more about the characters. Imagine if instead of being a tight-knit group, the five of them started out in different cliques, different groups, and barely knew each other. Zordon would select them based on psychological profiles or something, and they would start out not really getting along that well (a couple of them could even be enemies). So then we get to watch them learning to trust each other, learning to work together, and so forth. At the same time, they would have to deal with keeping their identities secret from each of their own circles of friends.
Also, think about each episode; the Puddies and the monsters continuously attack our rangers at the school, in their homes, in their civilian identities, even putting their families in peril on a regular basis. But nobody is ever like "I wish I wasn't a Power Ranger, then my family wouldn't be in danger all the time!" So get some of that drama in there. Speaking of drama, all six of them are top students before and after they become rangers, and being a superhero never interferes with their grades. So put some pressure on these kids; being a superhero is tough. Also on that note, Rita (or whichever villain) shouldn't know their identities (Zordon would have a way to protect them). What's the point of having a secret superhero identity if your enemy knows it but your friends and family don't?
I'm juiced up; I think I'm just getting started.
[Edit: And yes, I posted the above before I read everyone else's posts.]