Running into ranked the second you get 16 champions is generally a death sentence. You own quite a number of AP and burst champions, but no ADC's, no Junglers unless you count Tahm Kench maybe.
For an ADC, I'd highly recommend Corki, as he's both fun to play and very versatile, and has great outplay potential with his W, his rockets, and deals surprising burst with Q. As I've read somewhere before. "Corki is the definition of a Mid-Game Monster." He weaves spells and auto-attacks in a Space Opera Symphony.
Wukong is a great fighter that can go into the top lane or jungle, depending on preference. His damage is usually always relevant in all stages of the game, and his Shadow Monkey Jutsu thing is just icing on the cake due to it's escape and outplay potential. His Ultimate is never not useful unless the Monkey King is behind, due to the constant damage and AOE Knock-up.
If for whatever reason you need raw versatility, be it a tank, a support, or a jungler, I'd recommend buying Maokai. He's not all that hard to get used to, but his peeling potential and sustain, he's just a monster to fight against for your opponents.
Tree's OP.
As for your question about Vel'Koz, Zed, and Fiora.
Vel'Koz isn't highly item reliant, and the outplay potential being able to be done with him with quite high with all his CC and Geometry.. Plus, there is nothing quite as satisfying as melting the shit out of someone with a laser beam.
I don't own Zed, so I dunno about him.
Fiora. Oh lawdy Fiora. She can get a double kill from a gank using only a Ravenous Hydra or a Tiamat, and the comebacks you can make from her AOE Ult and Parry are unreal. She can go up against a majority of the game's laners because everything about her is just strong, not to mention fun.
Last Champion I suggest taking is LeBlanc, as much I hate to say it. I will never buy her simply because I always have a bad experience playing against her. Stupid mobility, stupid point and click burst, and a passive that basically makes you GUESS which LeBlanc is the one you have to kill, which makes me miserable whenever I blow an ult and ignite on her.