Since you like Katarina and Diana I would tell you to choose between top and jungle to be your second role... Voyboy is prety successful with Katarina top, and Diana is a good jungler too as you might have noticed from scarra's stream. By choosing one of those roles you're one step ahead since you already know the mechanics of the champions you're playing, and then you can look up for other options to increase a little bit your choices.
I'm probably talking nonsense stuff, but I think that the way you carry games playing top is more likely the way you carry playing mid.. You farm, roam and if you get fed you become a powerhouse and rofl above the corpses of the enemy team.. Playing as a jungler you will be trying to get your lanes some feed, making your adc/apc/bruiser become stronger by setting up kills form them and making the enemy team fear your presence... I play most top or support, but my most successful role in ranked is jungler just because low elo players don't know how to effectively ward, freeze lanes (one thing that I'm trying sooooo hard to learn hahaha) and avoid free dmg.. At 1300 in the br server (brazillian players r a little bit unskilled compared to na/eu... lets say that gold on my server is silver on your.. so i don't know if what i'm saying will work for you) if you ask your mid to let the enemy push and gank at lvl 3 you have around 90% chance of getting a kill, just because they r not expecting that agression so early in the game..
Sorry if my post isn't clear enough... My english is almost as bad as scarra's warding skills xD
