If the person is walking through the forbidden door and signing with AEW it can't be a free agent as they wouldn't be walking through anything.
It's not gonna be anyone from MLW because of their contracts.
That leaves NJPW, which I don't see any of their big stars leaving Japan for here unless it's like Jay White but I think he's locked up for a few years as is Will Ospreay.
As @Based God stated it could be Juice Robinson but he hardly qualifies as a top wrestler but then again Tony's announcements are usually all letdown.
It could also be Konosuke Takeshita, although he wouldn't be making his debut BUT has said he wants to come to AEW.
Guess we'll find out Wednesday.