Before Arkane gave him that rather creepy mask we all know and love/fear, Corvo went through multiple design iterations, much of it influenced by the fact that the game was originally going to be set in 17th-18th century London as opposed to the strange mystical-mechanical world we've fallen in love with. Here are some of the concept artworks released on Kotaku.Evidently he has indeed undergone several design changes. Most likely due to three reasons.
1. The original design invokes a little too much of the Thief series.
2. The setting was shifting away from the original 17th century setting, and could now afford to be more fantastical.
And 3: The 18th century was too close to Assassins Creed 3 (although there was no way they could have known at the time).
Personally, I really like some of the concepts, especially the thumbnails, especially F and F2 in the third picture. Although if you gave me choice between them and the mask we all know, I honestly wouldn't know what to do. ;)