They'll need OFers and I'm predicting that Harper will "figure it out" in NYC and become a monster for them.
Not necessarily. Theoretically, Judge becomes a Dave Winfield-type in RF. Great athlete, good defense, monster power. Additionally, Frazier takes over in CF by 2018-19 and pushes Ellsbury to left field. Both of these are extremely possible. Now, luckily there's the DH in the AL, so you can rotate Judge as a DH with Harper if you'd like. That's all a long way away and Judge, Frazier, or anyone else could flame out or shoot up. The NYY also have a number of high-floor outfield types, like Dustin Fowler and Mason Williams, who are at worst-case 4th outfielders. They have depth.
I think Harper would dominate the NYC market and reach global icon status as a Yankee, but I also think Machado could do the same at a position where they only have one possible long-term guy (Andujar) near the upper levels of the minors. Outfielders are easy to find, great third basemen are not.