I think the question of how to adapt Captain Marvel (Or I guess Shazam now) really comes down to what sort of audience demographic the movie s supposed to appeal to. I think there are a few ways you could do it
1. Make it a kids movie. This is just plain simple because Captain Marvel is a classically wholesome and soft edged character and the true appeal of Billy Batson's story has always been unadulterated wish fulfillment fantasies. I don't think you'd really need to do anything special with the source material, just make it very simple and light hearted and kid friendly.
The problem with this is that there really aren't that many none-pixar/live-action family films with broad appeal really being made today so chances are it would be insufferable for anybody old enough to already know who Captain Marvel is. That and, I'm not sure what sort of boxoffice prospects a live action PG film has these days. I guess stuff like Alvin and the Chipmunks net enough to warrant sequels, but they seem relatively inexpensive to me, not like a super hero film.
2. the second option is to do what most super-hero films do, which is tailor toward to ever precious 13-30 male demographic and all their lovely wallets. But the so-called darker/edgier/more violent characters (like Batman, Iron Man, Wolverine and the X-Men, and Spider-man) normally fair best in these situations both critically and commercially so I imagine that if Shazam would be adapted for this most lucrative of markets they would have to take quite a few liberties with the source material.
I'm not one to say it can't be done, but I'd be really surprised if anybody managed to take Shazam and make it simultaneously appealing to young men/ bare passing resemblance to the source material/ worth watch.
3. The third option, I guess, would be to make it a kitsch/camp post-modern throwback/send up to Golden Age comics with varying levels of tongue-in-cheek satire and self awareness. I could actually imagine this being really good, it my mind it might have the deconstructed, high-culture/low-culture appeal of a Tarantino film with that great grey "is it art-house or exploitation?" meta-textual flair.
Of course, this option is more than likely going to go right over everybody's heads, or at least that's what the powers that be at Warner Bros. would tell you 3 minutes in to the pitch-meeting. Much too obtuse and high-concept for any major studio the green-light, and it's not exactly a property you could shop around to anybody else.
Can anybody think of another possibility to add?