Draftsight(1) is a good Free Cad DWG compatible for Linux that aims developing plugins(2) for make money. I think that model is appropiate to developing a Free (Open source) Qcad software.
I use and love Qcad. I hope this help to making Qcad best.
Greetings
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1) http://www.3ds.com/products/draftsight/ ... -software/
2) http://www.graebertmarket.com/