If Strickland loses to Portman, he can:
1) Run for Ohio governor in 2018 (He could beat Mike DeWine or Jon Husted)
2) Run for Ohio's 6th congressional district again against Bill Johnson (Romney won that district 55-43).
(Trump may overperform Romney there due to his white working class appeal)
3) Run for DNC chair in 2017
4) Cabinet position in a Hillary Clinton administration
5) Liberal think tank
He's 74 right now and will be 75 by the time of the election. If he loses to Portman, this is his last hurrah.
He's really getting up in age, but seems to show no signs of slowing down. If he doesn't want to go into retirement, he can probably join a liberal think tank (wasn't he part of one between losing the 2010 governor race and entering the Senate race?). I'm sure the DNC would probably favor someone younger as chair. He
could beat DeWine or Husted in 2018 (the former, like Strickland, has been around for a long time), but he'd have lost two consecutive statewide races, while DeWine is 5-2 statewide, and the Democrats will probably want someone else to run (Tim Ryan or Richard Cordray). Clinton (if she wins) appointing him to her cabinet is not unreasonable, though I wonder what he'd get. Running for OH-06 may not be a bad idea for him, he still seems pretty popular in his old district.