Maybe MLS would be a good option but he has to accept lower wage and the wishes of his family to not move with him, not sure if that's the case but it might happen.
I think with the way of life he has right now, it is hard for him to accept a major change. The best move would be to join Al-Hilal but obviously this might break the wonderful attack line that team has.
I don't blame Cristiano Ronaldo a lot for not winning anything in particular in Saudi Pro League since he has a top opponent, Al-Hilal, but also his teammates aren't always fit.
With Ronaldo's characteristics, it seems that he will not want to move to MLS. He once made a statement when he wanted to leave the European League that the Saudi League is better than MLS and he speculated that the Saudi League will soon follow like the Turkish League and the Dutch League in the next few years and playing in the UCL he seems to not see a future in MLS so I think it is likely that he will stay in the SPL until his retirement and may reject the offer from Al Hilal because he respects the club he is currently playing for.