Like others said, it depends on the area and convention... larger cons like Otakon and AX are going to have the largest number of KH cosplayers. It's going to be even more because of the 3DS game coming out soon.
If you're worried about doing an overdone costume, maybe try an alternate outfit? I know Riku gets at least one new costume for Dream Drop Distance, you could try one of those.
Zenkaikon ~ Tanoshiicon ~ Otakon ~ Saikoucon