Probably a trade both teams turn down.
I think it is worth mentioning that 1st overall picks don't always develop into elite players. In a 10 year span, from 2005 - 2014 (decent sample size and far enough back to understand how players panned out), we have had Erik Johnson, RNH, Ekblad (currently but subject to change), and Yakupov. So a 30% chance of getting a solid, but not elite player and 10% chance of drafting a bust. I'm not knocking on Hughes at all - kid looks great from what I've seen - just pointing out basic stats to assess the trade.
I think I'd rather keep Pettersson as he already looks like a ~90 point player in his sophomore season. Once he fills out more and potentially gets better linemates (Podkolzin), I could see him as a 100+ point player - that's exactly what I'm hoping Hughes becomes. Taylor Hall is the real wildcard here as we don't know if he'd re-sign or if Canucks could fit them both under the cap. His age and physical play style aren't very attractive to a team that is still rebuilding.