Captain Cold could be nice. See what they do with him, as opposed to what Smallville did with Killer Frost, or a contrast between two villains, with KF here and CC.
I'm not super familar though, myself, though, other than Reverse-Flash, I'd be more than up for Mirror Master, as his powers sound cool, and fairly interesting. A unique challenge for the Flash, and could be very visually interesting for the show to pull off.
What about yourself? Any Flash villains you'd like brought into Arrow first, then Flash, or Green Arrow villains here, then Arrow?