There is no way Ronaldo will consider returning to Manchester United after his previous experience that led to him moving to Saudi Arabia. I also think he will rather accept a move to Sporting than return to United, and with the way things are playing out I think he might even retire at Al Nassr.
When we mention English Premier League, we're formerly referring to a competitive league where the best team comes out with top ranking. In EPL, Manchester City and Liverpool are reigning Champions and have become hurdles for these average clubs coming up to compete. If you're a player and you want to shine, stick to consistent performance and upgrade in all physiques, then watch when elite teams will come for bidding.
Cristiano Ronaldo is one of the undisputed G.O.A.T in football. He's not making a u-turn back to the English Premier League, you know why? He's having a great time in Saudi Arabia and have banged more goals than expected. Ronaldo is Manchester United legend, he's been in the system for years and know exactly how it operates. He's not frighten by challenges, rather he embrace them, that's the kind of player he is.