Melo is not being traded, so at this point I'd just accept it for what it is.
So how do you want a team built around Melo/Porzingis?
Ideally you have a forward in there who is quick, versatile, and strong. Someone that can play 4, 5 and even some 3. Because KP/Melo are versatile, they don't have a set position on offense or defense. If you can get a forward in there who can do the same, that's the way to go.
First thing I'd do is bench Robin Lopez. As good a signing as he was, I'd bring him off the bench. He'd be a great backup to have. Make our bench really, really solid. We'd have someone we could throw the ball into to score, give us energy and hustle off the bench as well.
Question is who is that player that fits in with Melo/Porzingis? My inclination is to go with Al Horford. With Al you solidify your starting front court as arguably league best, if not at least top 3-4. There's more speed, versatility, spacing, all around talent with that group. With RoLo and DWill off the bench, you can now have a 5 man rotation up front that is extremely solid. Downside is Al's asking price. Probably going to be a contract in the 90 million/4 year range. Maybe even more.
Even Horford/Porzingis/Melo/Afflalo/Grant would be a major upgrade over this season's starters.