The problem with Kogmaw is that I am not skilled at leading the opponent. As for Tristana, I haven't played her enough yet. She seems viable, though. I think my main problem is positioning, though. With Fiora, I enjoy shredding opponents with a few quick hits, then moving to the next. Plus, her ult allows me to survive a bit until my team comes while killing them. I want a ranged who can do that too, or at least rip people apart quickly. If positioning is my problem, then I need to find someone who can move around well, and still deal a lot of damage quickly.
Vayne might also fit, because she can shred anything (thanks to her true damage component), has Tumble to reposition, and Condemn to push them back. Her ult also gives Tumble some brief invisibility, to help with escaping and repositioning.
Other carries w/ reposition abilities are:
Ezreal (his blink - Arcane Shift I think?)
Draven (kinda - Blood Rush gives you movement speed, but it isn't an instant move)