If we look empirically at what have actually worked for us over the past ten-fifteen years, when it comes to buying proper talents, we should probably look to buy more players from the following teams (and/or their likes).
Ajax (Dolberg, Kluivert, Klaasen)
Atletico Madrid (Griezmann)
Brøndby (no-one from the Danish league would improve us at the moment, but there are as always a few good youngsters (Duelund/Ingvartsen/Cornelius) to keep an eye out for and maybe there are some in Norway or Sweden too, though Isak, who seemed the most coveted recently went to Dortmund)
Real Sociedad (no idea but we will be linked with players from clubs at their level in Spain like left back Durmisi [Betis])
Inter [Espanyol] Icardi maybe Kondogbia
Southampton (van Dijk, Bertrand, Redmond) and from the same shelf perhaps a Schmeichel at Leicester and maybe even Drinkwater (what is the general opinion of him on here)?
Hoffenheim (Sule) and from about their tier (Forsberg, Keita, Werner) all at RB Leipzig)
Newcastle (there are as always a few potential stars in the Championship at the moment, we just need to single them out)
I know there are tons more to pick from and that this is flawed but those are players from the top of my head that I know off that we could pick up in clubs (levels) we've done so with great success before.
Does this even make sense as a marker or guideline? No idea..