Celtics - Really don't see it happening. Don't think Lebron would want to "fit into" Stevens system. Whether Ainge would do it probably depends on how likely he thinks Kyrie will re-sign with us. Even if Ainge is concerned about Kyrie leaving, going after PG13 or Kawhi might be preferable and would certainly be less disruptive.
I hope it does not happen, I hate watching one guy dribble out the shot clock which he does often.