Mind nerfing Manamune?
Actually, maybe (since this is a better idea) just nerf Ezreal himself, or make Muramana unable to proc twice on his W, on top of the automatic proc you get from Q, E and your AAs.
All I know is that, as of now, his AD builds always involve Muramana because of how stupidly-busted he is with it in tow. It outclasses his AP build, it outclasses any other build you can find. He is a problem, combined with Muramana.
I'd much prefer to be able to build AP if I felt comfortable landing skill-shots and being squishy, in return for increased DPS. I'd much prefer to be able to build AS vs. tough teams full of tanks... but as of now, how powerful he is with Muramana drowns all of that out.