I'd trade Donaldson if the Twins didn't need to eat too much of his salary and/or give up too much with him as part of a package. But I don't think that would be the case, so I would have him play 3B about half of the time and DH the rest of the time.
If you can, and if the Twins fail to get quality starting pitching (almost a certainty given the history of their ownership), and if Josh is having a good year, try trading him before the All-Star Game.
And, of course, I would trade Donaldson for nothing if another team ate the entirety of his salary.