Darker, yes...but an epic battle of Good vs. Evil, including messianic overtones. In reality, I don't see it as any darker than The Chronicles of Narnia (with a less overt messianic message, if you ask me...in LOTR, you need to study, or at least read the trillogy more than once, to really see it).
Oh...and by the way...I see the same basic story and the same message (yeah, including the messianic overtones) in the Star Wars story.
That's my opinion, of course...and probably a bit biased on this subject, since I have written a number of papers on such in Lit classes a while ago (ok...about a decade or so...) and then defended them in class. I also wrote on Baseball in literature (which I got serious laughs in class...until I made the kids in the class think...I was 25 in a sophmore lit class)
I think any of these would be good for Disney to use...but the creatures in LOTR could probably be more upsetting to children.
Kevin