Rob McCarron at Wrestling Inc thinks that the WWE will go after guys who have been in the WWE before but were let go like Drew Galloway and Ethan Carter. He doesn't believe that the WWE will pursue guys like Okada or Tanahashi.
Okada recently said he will "never" go to WWE and his dream is to grow NJPW enough to where they could run Madison Square Garden.
Tanahashi is in his 40s, I don't think he's going anywhere.
But guys like Moose, Roddy Strong, The Young Bucks, ACH, Adam Cole... all of their contracts expire between now and the end of the year except Cole, who is signed through April.
They've long been rumored to be trying to sign Ricochet/Prince Puma
They've showed interest in Jay Lethal and the Briscoes before
The Hardys could be a target, and as you said Galloway and EC3. Maybe Eddie Edwards.
Go Shiozaki is out there
Not sure about the contract statuses of guys like Naito, Dalton Castle, Ishii, Goto, Omega, Rey Mysterio, Texano, King Cuerno, Mil Muertes, etc but they could easily be targets (maybe not Dalton Castle). Muertes is pretty old, so maybe not him
It's really a scary time for fans of the major non-WWE promotions. Only ROH and NJPW are financially healthy, and they're the ones that have been hit hardest by WWE lately.
And the rest of them? We honestly don't know if LU, TNA, or AJPW will still exist in 2 years. CMLL and AAA are in a lot of trouble, but they're pretty resilient. Noah isn't doing well either.
I don't blame WWE for signing great talent but it'll be sad to see WWE's only real competition take these hits.