If TNA were to close down, I think that ROH would see a bigger impact than WWE. Guys like Styles, Daniels, Shelley, Sabin, Joe, and Homicide would all be pumped back into Ring of Honor and really revitalize it.
WWE may re-sign Angle to a part-time contract, but that's about it. I could maybe see Nash taking some sort of backstage role, or else maybe even an on-screen authority role. Sting would pass on the WWE, and the WWE would pass on Steiner. Booker T would likely go back to Houston to run his wrestling school.
The interesting one would be Jeff Jarrett. I don't THINK he would come on in a wrestling role, but crazier things have happened. Vince would be smart to bring in Jeff as some sort of road agent to use him in trying to bring over at least some TNA talent. After that, though, I suspect that he too would return home. However, when TNA closes, that's when guys like Nash, Steiner, Sting, and even Jarrett will find themselves with DVDs and Hall of Fame spots for some of them right away, and all of them eventually.