#4 Rob Van Dam

Well, he may be 47 years old, but you'd never think it if you've seen him in the ring. Years of yoga and stretching have kept RVD in prime shape and the Whole Damn Show can still put on quite the show in the ring.
In fact, the former WWE Champion has even said that, while he wouldn't be keen on returning on a full-time basis - like Mysterio - he would definitely consider a return to WWE if the money was right and the idea for a final run were both right.
The main stumbling block here would likely be WWE wanting a big commitment and Van Dam not wanting to work such a frenetic schedule.
The ECW legend had a short, not incredibly memorable stint in WWE in 2014, but if used correctly, he could provide a huge boost to either Raw or SmackDown.