Say there's a main topic area that has many sub-topics. Could it work to register a domain name for the main topic area and have the sub-topics as sub-domains with a blog on each topic? For example, the main topic could be ice cream so the main site could be icecream.com. Then you could have chocolate.icecream.com vanilla.icecream.com strawberry.icecream.com and so on... Each blog on each sub-domain would deal only with that particular sub-topic and you could feed a combined feed back to the main icecream.com site. So each one could be set up for adsense and other ads specific to that sub-topic rather than just general ice cream. Does this make sense? Is it better to set it up that way or to set up iceream.com/chocolate; icecream.com/vanilla; etc.? Which would be better for search engine results do you think?
I actually have my forum site set up like that (your second example). I own MoparRegister.com, DodgeRegister.com, JeepRegister.com, and DodgeRegister.com. Since the three brands all have Mopar in common I just forward each site to MoparRegister.com/dodge, MoparRegister.com/jeep etc. I thought about having subdomains but I think the search results would be the same regardless.