Frenkie de Jong. We need to make another attempt to sign him, but not chase him all summer. Unfortunately for us, there aren't many players with his skillset. There are, of course, midfielders who are equally as good as him or better in some parts of the game, but he's got the full package that we should be looking for in a midfielder next to Casemiro. De Jong is world class in controlling the game, passing, progressive passing, dribbling, press resistance and ball carrying.
Caicedo, for instance, is not. He is excellent defensively and would be a fantastic backup for Casemiro but that's not gonna happen.
Alternatives to De Jong are hard to find due to his skillset, but if we're going to look at someone with similar skillset but not equally as good, I'd look at Enzo Le Fée from Lorient.
Statistically, he is an even better dribbler than FdJ and is an excellent ball carrier as well. The upside with him, compared to Frenkie, is that he is excellent defensively and a much bigger goal threat and also assists more. He's scored some absolute cracking goals from distance so he has that in his locker.
My worry with him is his pass completion (80.1%) . It is slightly worse than Fred which puts him in below average for midfielders. Of course, that could easily be improved and with his age, experience is only going to make him better. If it was to me, I'd go for Frenkie de Jong as first priority, and Enzo Le Fée as the number two on the list.