I have no interest in watching Hulk Hogan wrestle again. He was always something of a limited performer in the ring, but he can barely walk now. Any match he has, regardless of Hogan's name, is going to be extremely subpar and both he and whomever his opponent would be would come out of it looking bad.
I doubt that Hogan has any real interest in joining TNA, he's talked too much shit about them in the past. This is a tactic that he's used in the past in order to kindle WWE interest in him. I doubt very much that TNA could afford him to be quite honest. Hogan wants the moon and the stars but, ultimately, doesn't bring enough with him to justify it anymore. During his last WWE program against Randy Orton, he said Vince paid him 400 grand for the match he had with him at SummerSlam. According to Hogan, he threw the check away and considered it low. He made more from that one match than most of TNA's wrestlers make in a year, and even the ones that do earn more than that don't do it by much. I think Sting and Angle get around $600,000 a year in TNA.
If he did go to TNA, and it's a huge if, ratings would probably go up temporarily until Hogan worked a few shitty matches and fans saw just how limited he actually is. The nostalgia factor would play a role in that as well, but it would fade before too long.
However, Hogan could be a steady draw I think if he were to accept some sort of nonwrestling, but still on air, role with either of the companies. Make him a GM or manage a faction or even an on air commentator. That being said, I don't think Hogan would readily accept those positions. His ego is simply too big to accept the fact that the days where he could give a credible, competitive wrestling match are over and done with. If Vince were to begin negotiations with him, Vince should play hardball. Both can make a lot of money with the right arrangement, but Hogan also needs to realize that there's not any single "top guy" in wrestling anymore. That was another reason for his fallout with Vince McMahon.