I think Mark Warner would make a great centrist candidate, someone who's good on law and order and would restore faith in our intelligence apparatus, while taking a hard line on foreign meddling in our elections. He's got a track record of bipartisanship with a focus on getting things done. I guess he'd be 65 in 2020...not too old assuming he stays in decent health.
Unfortunately, Republicans will think he's too "Deep State", and Democrats will say he isn't progressive enough or he's in bed with business and military interests, so he probably doesn't have a snowball's chance in hell getting out of the primary.