1. Bobby Roode
He's the most entertaining guy not in the WWE right now. The guy is a superstar. Both in the ring and on the mic. He's the epitome of a heel. In my opinion, he's really the best thing TNA has going for them. At least the only thing they have that gets my attention.
2. John Morrison
I'd give him another shot to put it all together. He's got the look. The charisma. The moveset in the ring. I know he's not great on the mic, but that's a skill that can be developed over time and with more experience. And personally, by the time he left he had improved leaps and bounds on the mic than where he started. I think he can still be a main event superstar.
3. Sting
Because he's a huge name. WWE would make him a huge draw. And we'd see some excellent big time main events vs. The Rock and vs The Undertaker. In the end though, he'd probably only be around for a short stint like Goldberg in 2003.
Honourable mentions:
I'd want AJ Styles from five years ago. I'd like to see Austin Aries and Samoa Joe in WWE but I think they'd both just get wasted similar to Taz. Maybe Aries can get over in a Daniel Bryan fashion. And Maria Kanellis just because she's the hottest woman in wrestling today.
Don't care for Jeff Hardy. Or really for Kurt Angle to return. He's done all there is to do in the WWE.