I doubt McDavid would be a first line center his first season. It would probably be a scenario where the top 3 lines are rolled with a 1A, 1B, 1C scenario. Give Turris or Zibby the hardest matchups (depending how well those two lines finish up the current season).
Keep all of our core guys if possible....try out a few guys on the wing for an extended period of time (Pageau, Zibby, Lazar). If none of them look like they would develop into strong wingers, and we really have too many NHL caliber centers, then we can think of making a trade (probably Pageau, Smith, Prince). If McJesus is as good as advertised, we can also dangle the 2016 first round pick for a top 4 D.
Edit: We have to remember, McDavid might put up Crosby-esque numbers in the NHL, but it won't be for a while. He does not have the same body Crosby did coming out of junior. Not at all. McDavid is seriously under-developed physically at the moment (although that might change a bit by September). I don't expect a huge NHL impact from him next season.