So, when it comes to my opinion about a Bitcointalk forum logo, I think it will be hard to figure out something that is Bitcoin-related, but doesn't immediately get confused / understood as a Bitcoin or Bitcoin-fork logo.
When I think about a design element that nowadays I only see on Bitcontalk, it's this old-school, plain-HTML table-based layout with the matching gradient behind it.
Maybe someone can make a logo that incorporates this HTML design, because while I know it's just the old SMF software, nowadays this design really sets Bitcointalk apart from any other website.