I don't think all the current threads need to be scrapped. I was thinking that the topics could be proposed by users - the same way requesting a new thread works on this board. I mean the mods could think of some, and then if anything was missing, the users could suggest it.
As for spoilers - that's a tricky subject. When I was reading the books, I just didn't read anything online - I wanted to be safe. I think if people only stick to forums for the books they have read, and users remember to be respectful in the threads, they should be okay. But when I don't want things spoiled - I just stay out of forums - that's rule #1 for me!
I'm not sold on my suggestion - no system will be perfect - but we have to find something better than the current system. I just think if things are broken down by chapter, it will be tricky for people who don't have the book sitting right in front of them. I can't remember what happened in what chapter. Oooh - but maybe there's a solution for that. Perhaps whoever starts the topic on, say, chapter 6 - they could do a little synopsis of it in the opening post. That might work then.