MagicJuggler - depending on what you are looking for, a lot of it is probably already around the forum in one place or another.
- There is already a Warhammer Fantasy Battle Legacy Discussion forum (under the 'More GW games' bit)
- In GW Specialist games there are a few cool Epic threads - an Epic 30k thread etc. which has a few people participating. You also get topics on Man'o'war, BFG, Tyranid Attack etc. (just a few examples of threads I have seen recently)
If you're specifically after old edition 40k discussion that might be more of a struggle, as there is a lot of churn in the 40k sections and older 40k editions might not get as much traffic (so it will get pushed down quickly)
Agree though 'Oldhammer' has been around for a while, but is definitely picking up steam in the social media community (there seem to be a lot more contributors now than there were 2-3 years ago).
I wonder how much of it is due to things like the 40k universe changing so radically in the new editions, and there are people who prefer the older settings.