I'm saying Maicer / Kawasaki at 2B actually. I WOULD like to upgrade that, but I'm not sure it's as critical as other areas.
As for the SP, we are ranked 13th in WAR from our starters with additional help on the way sooner rather than later (and expected improvement from Hutch). If we can get a true staff ace, then fine. Otherwise, I'm not sure we can reasonably expect anyone who comes in to be an improvement over what we have.
At catcher there IS a perfect solution on the FA list: Russell Martin. Resigning Melky fixes LF, although if that falls through or costs too much Aoki could be interesting and probably wouldn't cost a ton. There's nothing obvious at 2b, but Headley at 3rd (coming off a poor offensive season that might make his salary reasonable) and moving Lawrie to second would work. I guess it all depends on cost and payroll.
For me though, addressing LF and C are the biggest issues. I don't see CF or 2B as quite as large a problem (again assuming Maicer comes back okay). Heck, Gose has put up 1.2 WAR this year in 205 PA. Extend that to 400 PA (probably the platoon limit) and we should be just fine in CF.