After some discussion about it, I started a Gundam GB. Given that mecha robot modeling isn't very popular here, it seemed best to leave it completely open. Bandai makes an astounding variety of kits and having it completley open is definitely a-kin to a "any WW2" GB. In my mind, the thing that makes it okay is the small number of people interested and that it's a pretty specific topic, in of itself.
As for the Badge thing, I'm in agreement with Rick about the badges. I see them as advertisement for the Group Build. Once the one I've started is done, I'll see if there's interest in another. If there is, I'll start that and there will be a different Badge that will say 2006 rather than 2005 and with a different picture. If they become popular enough to be done 2 or 3 times I year, they'll probably get roman numerals or somesuch.
I guess my thoughts are best summed up with this: If you're introducing a less common era or subject, keep it broad. If there's lots of interest in the subject/era already, you can tighten it up a bit. I imagine WW2 is probably the most common category possible.
You can also make badges clickable links to the thread in question. Go on, give mine a try! Here's the tags for the Text & Graphic profile:
[*url="The Link To Your Thread"] [*img]The Link to Your Badge[*/img][*/url]
Remove the * and put in the URLs needed to make it work.