McDavid has certainly got off to a hot start in his career, and I'm very excited (as an Oilers' fan / sufferer) to see where he goes.
It really could go in any of several different directions. Two scoring titles in a row in one's first three seasons is pretty amazing anytime, and especially now. Since he's just hitting his physical prime (probably) around now and entering his best years, the sky's the limit, especially considering he hasn't played with many great players yet. His style ('visually', as MacT would say) is extremely exciting. Should McDavid continue to trend upward for another year or two, say, and does in fact win three or four scoring titles in a row, and the Oilers improve a lot, we could start talking about his chances of potentially approaching a 'big four' level. (Potentially, mind.)
On the other hand, McDavid could just as likely go in a direction somewhere between Dionne and Crosby. We may have seen the best of him already, and he may run into injury problems and the like. There's certainly a chance the Oilers will manage to blow the window of opportunity they have with him. If he finishes between 2 and 10 in scoring for the next ten years and the Oilers make the playoffs only a few times (being eliminated quickly), he's basically going to end up a more decorated Marcel Dionne.
But why not think optimistically? He does have at least an outside chance of approaching big-four kind of territory from now... although the odds are strongly against it.