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Sporting CP would be a good choice if the superstar's ambitious goal is to return to the Champions League.
A move to Sporting CP would put Ronaldo in a position to return to his first club and show, at 40 years old, that he can still perform at the highest level of football. It would be the perfect situation for any player with such enormous ambitions. He would then keep himself onside with the Portuguese national team too, an outcome that surely outweighs anything else as far as priorities go leading up to the 2026 World Cup.
Furthermore, it would have to be weighed financially and also take into account his wishes and the club's strategy. Though it may sound far-fetched for Ronaldo to depart the SPL with all the perks he has earned, returning to Sporting CP to stamp an elegant endpoint to his career is not beyond the bounds of possibility.
But I am not sure whether that is what he really wants. He can go through some nostalgic moments then, I get that, but Sporting won't be very successful in the Champions League and it doesn't come down to a one man Ronaldo show or otherwise Al Nassr would be better off, too. He is still skilled and can score goals, which is what he proves a lot in the Portuguese national team, but I don't think he would be doing very well for Sporting in the Champions League. In the domestic league yes, but not so much in the CL as you need several superstars to reach something in that competition.