A Lion King dark ride? Meh... for some reason, the idea just doesn't sit well with me. Dark rides, to me, suit the much older Disney films better. The Lion King is an incredible story with epic proportions... will a simple dark ride really be able to do it justice?
I'd much rather prefer a Broadway-esque stage show at one of the parks, perhaps even as a replacement to Festival of the Lion King. It's already on Broadway, so they could easily tone it down a bit and turn it into something that's on the scale of, say, the Aladdin stage show over at Disneyland (if it's still there). I would enjoy that, personally.
Another thing I was thinking Disney could do would be a tame coaster, maybe on the same level as Big Thunder Mountain. The premise could be that we're trying to avoid the stampede of wildebeast with Simba and, after he is confronted by Scar, run for our lives out of the gorge and through the thorny forest with him to escape from the hyenas. It could end with Simba (and us, too) meeting Timon & Pumbaa, and that's when the ride would end. That would be pretty cool, too.