I agree that it's not set in stone however I want Icar to be accessible as I can. Now there are some bits of the system that are really very novel so I want the reader to feel like there are at least some familiar things about it.
That is exactly the reasoning I used. The main reason Midian has such things as armour classes and Elves is for player familiarity. Using classic gaming elements isn't always a bad thing, and it can help highlight the real features of a system or setting.