Galio works for obvious reasons.
Sion is a melee champion, a good cass would rape one. (I main cass..)
LeBlanc works because of her INSANE burst, but LeBlanc is pretty worthless late game (that's a whole other discussion), however, cass is INSANE late game so..perhaps the INSANE balances out? Carry your team through early/mid game and shut cass down and you'll be better off for it.
Ahri has plenty of mechanisms to dodge Cass's poke, she'd be a good pick.
Fiddlesticks. Silence. Poked me? I'll heal. Cass pissing you off and want her ganked? I'll fear you. Solid AP mid champion that isn't seen very often anymore, definitely has the tools to counter cass.
Believe it or not, Karthus. Karthus has even lower CDs than cass. Not to mention Karth is one of those champions that can help the other lanes while never leaving his, so if you assist in top or bottom taking their tower the sooner they can help you. He's also pretty much non-relient on early-game farm. (I know you just had a heart attack, let me explain) if you miss a lot of early game farm all you have to do is press R and rack up assist/kill gold. Not to mention end game in the middle of a team fight your E, Q, and R do insane amounts of damage making you one of the best in any team fight. Not to mention you have decent initiation due to your passive.
I hope this helped, good luck in mid. If you'd like to 1v1 me in mid sometime and discuss strats I'd be more than happy, add me in game. <3