I just randomly chose corki for the heck of it. I realize why no one plays him anymore. His base damage is very low, and abilities don't do much for mid early-mid game at all. He really needs this buff its sad to see this ad carry not be able to influence the game at all.
Maybe you just played him wrong/badly?
I can't tell if this is true or sarcasm. He isn't very strong, but he's not weak in the mid game. If anything, his early mid to mid-game power is still very good. I think his biggest problem is item synergy. Trinity Force is better than ever on him; however, after it, his build varies and falls off significantly.
Old Tri-Force had crit and only 10% CDR, which I thought built nicely into Essence Reaver, Rapid Firecannon, and IE. With the new Tri-Force having no crit and 20%, his power spikes a bunch after its completion, but his build can go in like 3 directions after it. A lot of people build Hextech Gunblade on him now, but I don't like that very much.
I like building Sorc shoes, Tri-Force, Bloodthirster, and Maw of Malmortius. Then, I fill in the rest with GA or Void Staff or something. I'm missing his 50-50 Physical/Magic crits though... 
And a fresh coat of paint.
