Cat-themed Malzahar--he could be a floating guy dressed up like a cat. Voidlings would be kittens, Q wouldn't really change, but maybe a "MEOW" could play on animation. W could have cat food particles, E could be catnip, R... not much different. Credit goes to "Stiles Bot"
This does not sound like a good concept. A good concept has at least one strong link between the champion and the skin - something that makes people think to themselves "Why didn't I think of that?" For example, a Pool Party Thresh skin would play on the lifesaver-like function of his Dark Passage skill. There is no such strong link between Malzahar and cats.
