It's more challenging to make one that isn't so obvious. Castro is so far out in left, it's not even on the planet. Sanders has said that if they want him, he'll run. Didn't say as which party. I'm sure cons wish he'd run as INDEPENDENT. Don't think he'd do that. Cons may be skeerd Paul will run as one. But really, what con ticket could beat Clinton/Sanders? I really don't see one.
You are theorizing about someone as a potential VP candidate, which - of course - will never happen, because he is just too offbeat to be real. Sanders is a coward, and a spineless typical socialist who ALWAYS caucuses with the Democrats, freeloading off them, but never actually wants to be called a Democrat.
Let me theorize about a Republican VP candidate, whose name I have never heard from anyone as such, but would be - in my opinion - a valuable addition to ay Republican ticket: Judge Janine Pirro.
I remember when wingnuts were saying that Carly Fiorini was a shoo-in. As was Sharron Angle.
I remember when wingnuts were (and are still saying) that Hillary was a shoo-in too....