I think having something semi-specific is the best route, personally.That way you have a commonality, but still a good variety of builds. For example, there have been several GB's made around a specific country. So in these you can see a cross section of technological/political development in that country, but still have people building several different things. Another example (and this will sound really self serving, but it's not!) is the 1942/43 GB's that I have hosted. Because WW2 was such a broad war, it makes for lots of subjects, but using the year as a basis ties them all together and gives a sense of what was going on all over the world at that given time.
More than anything though, I think the people in the GB make the biggest difference. I have really enjoyed the 42/43 GBs because most of the builders have included detailed stories about their build subjects, effectively teaching everyone (including me) about historic subjects that we did not know about before. On the other side, I have really enjoyed the Sweedish and French GB's that Vance hosted, just because the folks involved with those enjoy friendly small talk in between build posts, giving the build a much more social feel.
That's my thoughts, sorry it's a little long!
Budd