Back on topic, no I can't see Ash with a legendary Pokemon. Just as Ash was unquestionably dominating at the beginning of Johto thanks to his Charizard, it would be incredibly cheap to allow him to carry on the same action with a legendary Pokemon instead. (I'm looking at you, Tobias.) I find Ash struggling on his journey far more interesting than him winning all the damn time. How boring would that get if he won every single gym battle on his first try? There isn't any fight or obstacle. It's like a walk in the park. The park may have some nice scenery and some children to stare at from the distance, but it's still just a park and an easy walk where nothing happened. Walking in the park and meeting a pack of cougars (yes, cougars) fighting a herd of bears (don't judge me) is far more interesting, which counts as an obstacle in your walk. I'd imagine walking over corpses to be very rough indeed.