[Retroman]

A thread devoted to a soap would be less beneficial in my opinion...
As some people may still post outside of that topic regardless, because there won't be any written rule with it saying you HAVE to post in that topic.

Plus it could disappear off the page, which again could mean people will post seperate topics about that soap. Or even forget the topic exists altogether.

A subforum would be a good idea, since you can write a little about it underneath telling people to post in there only, for that particular soap.

So I back the idea =]

Unless you tried stickied/pinned topics instead.