Carmelo's potentially an interesting value buy as a Plan B or C over the summer if free agency doesn't work out for Boston. Given that clearing out true max space for Boston is a moderately difficult task (particularly if they keep their draft pick and they intend for Yabusele to be in the NBA next year) there are some interesting things you can do with this team operating over the cap as well. Matching salaries for Carmelo would be a breeze, as if you're operating over the cap because free agency didn't work out you've got Zeller's expiring contract and full Bird Rights on Amir and Jerebko to work with, and Carmelo shouldn't cost very much in terms of assets given Jackson's desire to see him moved and Carmelo's NTC. As an added bonus, operating over the cap allows you to retain Olynyk, who's been useful off the bench, whereas signing a free agent basically means Kelly needs to be renounced (among other moves).
To be clear, I'd prefer to sign a free agent like Hayward, Griffin, or probably Millsap, but moving something like Zeller and Amir (as a S&T) packaged with some low-value picks (definitely not the Nets picks, and probably not the Grizz one either, but I'd talk about some mix of the Clips' first/Boston's own 2018/seconds) and maybe a low-value player or two for Carmelo while retaining Olynyk and signing Jerebko to a sizable 1+1 deal to be used as a building block for future trades isn't a terrible option. He's pretty far down the list of potential targets (in no particular order I'd prefer Butler, George, Hayward, Blake, Millsap and - if he ever became available - Klay Thompson), but Boston could use another player to take some of the scoring load off IT's shoulders for nights when he doesn't have it going, and plugging Carmelo into the Gerald Green/Amir Johnson spot in the starting lineup could be a viable fallback position there.