There are many different ways to set up next year's line-up, including the one you mentioned.
To me, adding a TOTR pitcher is essential for us to have any chance, but that is just the starting point to fixing our roster. We do need another solid SP'er and should not count on Gio as an addition, since he can be viewed as just replacing Pivetta. A trade makes sense on too many levels to not happen. We have various logjams that can allow us to trade from strength and depth, without really even noticing the loss.
Trading Casas and moving Devers to 1B is one idea. Mayer at 3B makes some sense, assuming Story can stay healthy. Moving Story to 2B and playing mayer at SS might work, too, and having casas and Devers in the line-up would be a plus, as well.
Trading an OF'er makes the most sense, to me. Abreu would not bring the same return as Anthony, but if we signed a guy like Burnes, trading Abreu plus a couple mid-level prospects might bring back a good enough SP'er to make the rotation close to whole.
The heavy LHB nature of our everyday players has to be dealt with, as all of the big 3 prospects are LHBs, too. (I'm not sure Campbell can be counted for the 2015 26 man roster, but he is a RHB.) Trading Yoshida seems like a pipedream, and we'd certainly have to pay a sizeable chunk of his contract to do it. I do love the idea of an Abreu-Ref DH platoon (back-up OF, too.) LF Duran, CF Rafaela, RF Anthony looks impressive.