I hope this stays away from controversial topics such as religion/abortion, at least at the start.
Throwing topics out off the top of my head here, several of these probably wouldn't stimulate a long debate
-Censorship, particularly by the government and/or directed at children

-How implementing cheap, effective desalination would affect the world

-How by trying to eradicate viruses we are forcing them to evolve into super-viruses.

-The future of healthcare and whether the U.S. should have a public option

-Should special interest groups have as much power in politics as they do today?
By the way rapezilla, the idea is that the seminars will be led by people who are consistent discussion posters and can put in the time and effort to research a certain topic. In other words, it won't be people who just quote a big article and slap on a "discuss" tag.

I like those, if the populous' opinion weighs on your choices, Hyde.

-(Il)legality of Marijuana (ofc)

-Where the Human Species will be in 500,000 years evolution-wise(neglecting the possible self-destruction, etc)

-How the we would react to a planetary crisis (Such as a meteor)

That last one seemed a bit shallow :P.

Anywho, nice idea Hyde. Discussion has just sort of been sitting on the same few topics for months now. About time something spicy gets thrown on the table.