Soldato
Vergne has been linked with a return to F1 (presumably at Toro Rosso) but he stated rather emphatically at Monza that he'd rather be at or near the top in FE than trundling around towards the back in F1.Agree. His best chance is going off to WEC or DTM.
I would say Formula-E, but that's where careers go to die.
I'm not sure if that says more about the state of F1 or his own ambitions myself.