Sure we're getting NWO version 8 million, and Hollywood Hulkster, and mick foley, but it's not really going to bring in new viewers on a week to week basis. Jeff Hardy in a main event program with Styles could do it, but who else could they bring in (realistically) and have people actually take notice.
I think if a Randy Orton or John Cena (not likely) came in, THAT is what would cause headlines...not an old man who'se best days were in the 80's and 90's.
Your picks?
Brock Lesnar. The only person without a WWE contract with has enough cred to really change the game.
I do believe their is enough older wrestling fans that would still take notice if Goldberg showed up in the iMPACT Zone. I have no idea what he would do but I think it would make a decent splash. Rock and Austin are the only ones however outside of major WWE contracted guys that I believe could push them to a new level.
I would honestly think the biggest Impact would be made by a Brock Lesnar. I know it's highly unlikely and would be very expensive to TNA, but TNA doesn't seem to have a problem fitting the bill for tons of other pieces of dead weight, they could probably get rid of a bunch to pay what he would ask.
I could see a very SLIM possibility of Brock trying to retrain himself for MMA and realizing that after his bout of Diverticulitis, it may not be the best idea. I had a foot of my colon removed in 2005 for the same reason(I'd developed Diverticulitis which had, in turn, perforated my colon) and I can see where he may have some problems.
So, say he decides that wrestling is where it's at where yes, he'll have to take bumps, but that area will be a fairly large target to any MMA opponents and could cause real damage. So then he decides that TNA is where it's at, being as the schedule is lighter and he may indeed have more impact there.
So he comes in. He debuts and cuts a promo saying that he left WWE because of the sports-entertainment aspect, he wants a real challenge. Everyone knows of his injury and he had to reassess things afterward, so decided to come to TNA. Here, they have a real chance to differentiate themselves from WWE by focusing on the athletics, etc.