Not likely to happen IMO. He's a WWE guy, I can't see him going to TNA.
Rey Mysterio doesn't need TNA. If he was to retire from in-ring competition in WWE, then I don't think he's that desperate to continue wrestling that he goes to TNA. I see him staying on in an ambassadorial role with WWE, making one off apperances and having one off matches every now and then. Another alternative I can see happening is him working a part time schedule similar to that of which HBK or Undertaker have done in the past.