You should check out some ROH and PWG for indies and New Japan for the international big leagues. Laid a lot of the foundation for American major promotion wrestling going into the 2010s and 2020s. ROH put on the show that led to the birth of AEW. I haven't seen much PWG only because they don't do live PPVs.
The below who appear to be part of each companies weeklies and/or PPVs (someone correct me if I am wrong) all had extensive time in ROH, PWG, or NJPW before debuting or redebuting in AEW, WWE, or Impact (dark matches excluded)
In AEW: Adam Cole, Bobby Fish, Brody King, Christopher Daniels, Danielson, Punk, Danhausen, Kingston, Ethan Page, Kazarian, Adam PAge, Lethal, Gresham, Keith Lee, Omega, Kyle O'Reilly, Young Bucks, Rush (I assume he will be featured a good bit going forward), Samoa Joe, Scorpio SKy, Brian Cage, Malaki Black, Pac, Penta, Fenix, Guevara, Andrade
In WWE/NXT: T-Bar, Shelton Benjamin, Rollins, Riddle, Owens, Damian Priest, Cody Rhodes, Ciampa, Cedric Alexander, Tozawa, Styles, Zayn, Ricochet, Viking Raiders, WALTER (aka Gunther), Roderick Strong, Cameron Grimes, Bobby Roode, MVP, Finn Balor, Nakamura, Drew McIntyre
In Impact: Alex Shelley, Chris Sabin, Doc Gallows, Eddie Edwards, The Briscoes, Josh Alexander, Karl Anderson, Kenny King, Matt Taven, Mike Bennett, Moose, Rich Swann, Sami Callihan