First, I admit that I'm not sold on DeRozan, especially for his $9.5M contract. His scoring is nearly as inefficient as Bargnani's, he's a terrible 3pt shooter, he can't consistently use his dribble to penetrate and his defense is below average. However, I'm not suggesting this just to get rid of DeRozan; I'm recognizing that in order to get the quality of starting PF that I want, the Raps are going to have to giveup legit talent in return (and talent that matches salaries too!).
Second, I think if a legit starting PF was added to provide a low-post presence, the SG (DeRozan or otherwise) wouldn't be needed as a top-3 scoring option in the starting lineup. Gay, Lowry and the new PF would be the primary scorers, with Valanciunas cleaning up the glass and being the defensive anchor. To me, in that revamped lineup, the starting SG should be a lockdown perimeter defender, provide good ball movement, not require lots of touches and not need the ball in his hands to score. I think Fields would be a great fit in that sort of revamped lineup, similar to the role that Tony Allen plays in Memphis. His shot seems to be coming around, so if he can improve it to the point where he can reliably hit open 3pt shots that come as a result of Gay, Lowry and the new PF penetrating and/or drawing double-teams, then I think he would be the ideal starting SG (similar to Anthony Parker during the playoff years with Bosh).
Ross provides a scoring option off the bench, as the 3rd wing. Anderson could be kept for another season, though I think a 4th wing would be a top-3 need for the team going forward, as I had indicated in my original post.