I think Lebron is unlikely but his move to LA increased the chances, since with Cleveland he would probably have played more games and minutes.
When do they have to name the team? If upcoming free agents have to confirm availability before signing contracts, that would rule out a few big names.
If they take 12 of: Barnes, Beal, Booker, Conley, DeRozan, Drummond, Gordon, Griffin, Harris, Jordan, Love, Lowry, McCollum, Middleton, Thomas and Turner, I think they would drop a game to the Boomers. They'd probably still go into the World Cup as favourites, but you'd take the field for the first time in a long time.
However, I think we see at least Davis, Westbrook, Oladipo, Wall and Lillard (not free agents, don't have crazy playoff workloads year after year). And that's probably enough for them to win it.