I heard Juve is going for De Gea and a couple other United players this January. Could this be an opportunity for Ronaldo (and his agent) to engineer 'a return'? 
Juventus seems very complete in all positions to my eyes. I wonder why they are going to buy more players, still Manchester United is a good market for buying some quality.
As for Ronaldo, It would be interesting to see them returning to Utd, but I don't think that he is interested in that move.