Some very obvious names you could pick out, but trying to think pragmatically, which players might've taken us to the next level and helped us to major trophies, that were very gettable at one time or another.
Always felt under Rafa we had a superb spine but lacked quality on the wings. Dani Alves as a marauding full back certainly would've helped. Imagine Robben and Ribery on the wings at that time with Masch, Gerrard, Alonso and Torres for company!
David Villa was linked for ages but never happened, he might've been better to have around the place than Voronin and Ngog.
Going a bit further back to Houllier, I reliably believed at 11 years old that Harry Kewell was the missing link when we signed him, and he may well have been if he stayed fit. Robert Pires of that era might've been enough to win us a league.
I remember us being linked to Ronaldo and Raul during Rafa's time. Both were past their best by that point (although Raul did have plenty more goals in him tbf) but it certainly would've been really cool to see. Having Morientes for that short spell was cool enough!
More recently it's harder to say because I watch less and less football outside of watching us. At varying times in Klopps era weve been both one centre back (the Kabak year) and one centre midfielder short (last year) which probably would've seen us put proper challenges together instead of having a couple of lost seasons. Not sure who'd have been the perfect fit recently for those positions?
There we go. Alves, Robben, Ribery, Villa, Pires, Ronaldo, Raul.