As I am as well a Q2A user I want to give out the information I think is the most helpful.
It includes some investment, but not as much as You may think.
First learn about buying expiring websites / domainnames. There is plenty of information in the web.
Second search one with high count of incoming links and/or traffic, ideally related to Your websites topic.
Buy it.
Than, set up a new page with some valuable information.
Than, add a questionform like diskussed in several threads here allready.
This form sends the question to Your q2a site.
Done.
Now You can see how questions are dropping in.
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The directory idea as well is an excellent one, if there is interest I would set it up with an original phpld license I still have unused.
Just let me know. Would be free for everybody, obviously.
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Than, I asked a similar question here:
http://www.question2answer.org/qa/1967/
It would be a feature through which siteowners could offer an extended search through "friendly" Q2A sites.
I hope one can understand what I mean, but this I think would be really a good one.
Any comment or related ideas are very welcome !!!