Yeah to get Cousins we would really need at least one "star potential" young player like say Andrew Wiggins or a draft pick that is in the top 5 area. The fact that Cousins is a young big man means almost every team is in play for him, because why not? It's hard to think of a lot of teams that absolutely would not have space for a guy like that.
Things that help the Celtics:
1) You can eliminate western conference teams from the list of suitors, it would be foolish for Sac to deal him within their conference.
2) He is friends with Isaiah Thomas, isn't he?
Other than that, I can see teams like the Sixers, Heat, Hawks, or Bucks putting together a deal for him. The Hawks can offer Teague, though he may not want to re-sign in SAC. The Heat could do something like Luol Deng + Whiteside or Winslow. The Bucks could offer Monroe who isn't great but is at least under contract and is an OK big man replacement.
I think that at the very least the Celtics would have to include Smart in any offer. Sully and a pick is not going to get it done.