I know that there's a high demand for a pure .NET FIX engine in the industry because there's a lot more front end programmers than the system infrastructure people, and people want a front-end FIX capability... you'll be surprised... seriously)
I'm sure this is the case. Just like I would love to have a group that comes over and cuts my lawn and takes out my garbage for free. It would be extremely effecient for me and would be in very high demand all around.....
The problem though is your ultimately just doing someone elses grunt work for free. I don't see how you escape that trap if you try to turn this into an institutional product.
Quantlib I'm sure benefits from the fact that its interesting work to do and share...this is just grunt work that I don't see how you will get people involved if its not purely out of their own individual self interest for their own projects.
As far as retail APIs, if you really want to get crazy I'm sure there is opportunity in some kind of open source retail version of FIX, maybe even a small license fee. I mean lets face it, the retail API situation is a complete joke and far from effecient.