This is more hypothetical future of hockey than history of hockey, but I think that McDavid probably ends up a 9.5 player to the big four being 10s. Possibly the clear cut #5 assuming some kind of normal career arc and not Howe-like longevity and consistency. I do think that a Howe peak is in play, but the level that Orr/Gretzky/Lemieux showed they were capable of looks beyond him.
This is increasingly how I see it. The gap between the Big Four and five is strong enough that all but two posters in the last list adhered to it. And only one poster in the 2013 list. There's about a dozen candidates that could be argued at five, which says to me there's room for someone to come in and be better than the field but worse that the Big Four.
McDavid is on his way to his fifth Art Ross, third Hart, and fourth Pearson. He's putting up a resume that is comfortably making him the choice at 5.
He has not been playing at a peak level of a Gretzky/Lemieux/Orr. He is peaking at a level comparable to Howe. It's near impossible for him to keep up to the overall resume of Howe. Howe had 10 seasons of being top 2 in Hart voting amongst forwards. McDavid is about to do 4.
But being the consensus choice at 5 is realistic.
Right now, I see the following being popular choices for 5 (listed chronologically)
Richard
Harvey
Beliveau
Hull
Hasek
Roy
Crosby
That's going to be 95% of the picks for 5. You'll get a few Ovechkin or Jagr or Bourque too.
Looking at forwards only, can McDavid build a resume clearly above Richard/Beliveau/Hull/Crosby? I think he's in the process of doing so.
Being realistic, it'll take a Smythe and Cup to make it consensus.