Casemiro and Ericsson are weird choices, since neither have hugely declined relative to their peak. Eriksson is a solid but unspectacular player, like he always has been He spent his career at Spurs and Inter but was never the stand-out player in either side. Good player, not amazing. Essentially the same as he is for Utd.
Casemiro has always been a top class (albeit slightly limited) defensive midfielder and is doing this same job for Man Utd. He has maybe dropped from a 8.5/10 player to a 8/10 player but he is still one of the best in the league/world. I think there is a slight overrating of his peak going on by Utd fans at the moment. Yes is he world class, but if you go back 5 years and ask Utd fans to chose a single Real Madrid midfielder to sign for Utd then everyone would pick Modric first, and would be 50/50 between Kroos and Casemiro. And no-one is pretending that Kroos is a generational talent.
Speaking of which, the obvious recent choice for "player who should have signed for Utd younger" is Schweinsteiger. He was actually a generational talent, at a time when Utd really needed a top class central midfielder. Certainly better than Kroos, obviously better than Casemiro. And dare I say it, maybe even better than Scholes? And yet his Utd career was a complete shit show, and the less said the better
The other clear choice is Henrik Larsson. Criminally underrated in England, and yet joint with Thierry Henry as the best striker to ever play in the UK. Much better than Alan Shearer (who is apparently "the best English striker of the modern era" despite playing in a piss poor league and not having a single medal in his cabinet), and both Chris Sutton and John Hartson have confirmed that he is the best striker they've played with (above Shearer and Bergkamp respectively). Which is obvious to anyone with eyes that watched them both. The choice between Larsson and Ibrahimovic is difficult I agree, but Larsson slightly edges it for me due to being more versatile (Ibrahimovic only works as a focal centre and cant seem to integrate his game around other great players which is why Guardiola ditched him and he won nothing outside of bullying Italian sides, but Larsson showed at Barcelona he can play as part of a functional team).