I've been thinking long and hard about this - even fought with Sanglorian about it - and I've come to the decision that Icar Version 4 will hold a CC license. I am looking at BY-NC-SA (attribution, non-commercial, share-alike), so that others can make Icar stuff but not sell it without permission.
Why have I changed my tune? The more I think about it, the few times in the past people have offered stuff to be added to the canon, the ideas have been pretty good. Particularly those steeped in Icar-ness (Byrn, Fish, etc). Rather than stuff everything into a core rulebook each time, I should allow those people to make their own add-ons. It's not going to happen very often, so the sheer volume of Icar stuff I produce will always make the other stuff look like a footnote. I won't have to worry about quality because 90% of the Icar stuff will be by me - which will be of the quality I decide.
Version 3.5 won't have a CC license because I'm forcing myself not to open it up again. If I went back and produced 3.6, it would mean I wasn't working on 4 - which is bad, very bad.
Sanglorian, you may consider this as your first win! Victory laps and goal celebrations (or spitting if you're keen on baseball) are all acceptable.