As far as I am concerned, the problem with the Thumper forum are the megathreads. They are filling up the first page, displacing all new and "small" threads, and are a major pain in the ass to find anything in.
We have all that technology to subdivide stuff into sub forums, yet we pack all info concerning my beloved 640 ADV into one thread without any organization whatsoever. True, there is Meat Popsicle's super TOC for the LC4 ADV (Thanks for that!), but search is not that useful: You find the question, but maybe a relevant reply is 10 pages later?
-> Create a sub forum when you see a mega thread forming. That way information stays organized without spamming the general thumper forum too much, keeping it interesting and still diverse!
Why divide it up before it is needed? Divide when the need appears. A 690 adventure being announced? Create the 690 adv sub forum (or rather use the then existing LC4 690 sub forum I guess). Lots of new threads specific for any other bike? Create a sub forum for that. Of course the decision when to create a sub forum will be somewhat arbitrary, but that way the general Thumper forum will stay interesting.
I think there might be a more scenic route somewhere...