First of all, Aries, King, and MVP, have all left TNA. Pretty soon, Angle will be gone soon.
Lashley may be lightyears ahead of where he was in 2007, but WWE already tried with him. I like Lashley, but with all of the stars they have on not only the main roster, but also in NXT, why even waste the time, energy, and money in re-signing a guy who already left the company after a subpar performance? Again, I like Lashley, but there really isn't enough upside for WWE to resign him at almost 40 years old.
On the other hand, we have ECIII. Yes, he was also in WWE. He was much less successful in WWE than Bobby Lashley, and much like Lashley he has gotten exponentially better after leaving WWE. However- ECIII is seven years younger than Lashley. And I believe he is much more talented than nearly anyone on the TNA roster. ECIII is a guy who is different enough that adding him to the WWE or NXT Roster would be a much greater addition, and there is still a lot of money to make with him.
As for everybody else- I think Bobby Roode should have been signed ages ago. If anyone in TNA has WWE potential, it's Bobby Roode. And given Roode's ext tag partner Storm just showed up in NXT, there's a chance Roode may be given a role similar to Storm or Rhyno in NXT. But Roode won't be going to main roster WWE. I love the Wolves, but WWE just sees them as another couple of small guys, which they have plenty of. It may not be fair, but there isn't a high chance of the Wolves signing, considering they had a tryout just about 2 years ago and weren't signed. EY and Awesome Kong are both very good, but neither one of them is good enough that signing them would be worth taking a spot away from someone else.
And without a doubt Angle should resign. Given WWE's reliance on part-timers since 2010/11ish, I thought Angle should have been resigned years ago. Angle is without a doubt a future WWE Hall of Famer. Why not bring him back for a few matches? Not only is Angle a bigger star than basically everybody in TNA (excluding maybe Jeff Hardy), Angle is a bigger star than a majority of the WWE roster.
I say that the Hardyz should also be brought back for a short run, like the Dudleyz. If WWE were smart, they would sign Rockstar Spud, because he has loads of potential. There's hope that Spud would be a good addition to NXT. But they would have no idea how to properly book him on the main roster. Maybe resign Gail Kim for the NXT Women's Division, but that's about it. Nobody else on the TNA roster is worth looking at for WWE.