Video game website Joystiq posted an article about the Wii port of Cave Story, revealing the known Yellow costume being for Easy Mode, and an addition of a Hard Mode which gives a blue costume and blonde hair; as shown in the screenshot, as well as only 3HP. The article also quotes Pixel as stating there being "a LOT" of DLC planned, with possible requests, and three remixes.
The blog post also gives a bit more info.
Source: http://www.joystiq.com/2009/12/24/cave-story-coming-after-the-new-year-with-new-difficulty-modes/
http://www.nicalis.com/blog/2009/12/24/happy-christmas-eve/