The question should be, is Ronaldo coming back as a player or a coach? cause at his age I don't think Manchester United would allow him play for the team regardless of whatever he's helped the club achieved in the past, maybe in future after Ronaldo resigns from playing then get a good coaching badge to coach Manchester United, then they could consider bringing him coming back but as a player, I don't think so. Ronaldo himself knows that coming back to Manchester United for the 3rd time won't be possible, not at this point when United is trying to build up with younger talents his only high chance of returning is as a coach and that's if he proves himself, just like Xavi Alonso has done and returned to Real Madrid as a coach.
but I think Ronaldo returning to Manchester United to become a coach is very unlikely, and only limited to visiting because Ronaldo himself remembers himself being at the club and it is natural that there are many moments that he felt there, and seen from his statement when he was asked about when he stopped being a player then whether Ronaldo wanted to become a coach and his response was less interested in training the younger generation. because with that statement it is clear, about the attitude taken by Ronaldo.