Here are the teams that LeBron could join that I think he makes into a new contender via a sign and trade, and makes the team a lot more interesting -
Portland - Lillard and LeBron would create Kyrie/LeBron in a lot of ways. This would be a totally unanticipated move and inject some energy in the league. Portland has die hard fans. This would be cool for them. Could they get into a sign and trade to ship McCollum and matching contracts/picks to Cleveland?
Denver - The best team to not make the playoffs in a long time and a victim of Western Conference depth. Jokic and LeBron would be the most intriguing big men passing combo in NBA history. Denver has a strong mix of vets and young guys. You would think they could put some type of package together for Cleveland.
Utah - Mitchell + Gobert's competitive intensity + LeBron would just be fun. LeBron loses when he plays in Utah though, so not sure what's up with that. Hard to see the Jazz and Cavs making a deal work though. But man, if they did?
Indiana - They have some cap room to start with and have a LOT of roster flexibility in the future. Oladipo + Turner + LeBron is a core that could compete. Do they have anything Cleveland would bite on in terms of contracts and assets?
Milwaukee - The idea of two freaks like Greek Freak and LeBron on the same team is worthy of drool. The Bucks would have to ship out Middleton and some other valuable players, but LeBron and Freak are so versatile you could fill out the roster with all kinds of misfit toys and it feels like it would work.
None of this is going to happen. If the objective is to move the needle and disrupt the obvious future contenders, these are the 5 franchises that go over the top with LeBron in my eyes.