I know hirez is working on some Zhong Kui changes, and as a Diamond Zhong kui, who plays him mid in ranked, this makes me quite fearful. Zhong is a very unique mage that unfortunately gets little play time. Not because he isn't decent, but because people try to play him as your average mage.
In my opinion he only needs 3 small changes to be in a good spot, they are as follows:
ZK needs to have a speed boost during his Ult. Some would say he needs a slow, but since his 1 already slows, and people usually build Gem of isolation on him, a slow on his Ult would be redundant. A minor speed boost during his Ult will help him stick to targets or catch up if they use an escape.
his 3 no longer removes Card, and double the stun/level. Having him remove the card with his 3 is silly, especially in a fight. It reduces the potential damage if you use it too early, but if you wait too long, well the target may be out of range. Its just awkward to use. Not to mention you lose the ability to heal off your target if you do that. Just double the stun per level and remove the ability to remove the card.
have his skills cast instantly. To many times do i use skills only to have them get cancelled during the start-up. This is especially true for his 2 and 3. In this game every millisecond count, and the start up of his skills puts him at a slight disadvantage. Removing the start-up would be a sufficient enough buff to his skills for him to be a serious contender.
Thats it. Those are all the changes that Zhong Kui needs to truly be a viable mage in mid lane. Sure he wouldn't have the stopping power of Scylla or thoth, but he would be good enough for people to take him seriously as a mid lane God.