Like others here, I have a general theme / direction to what i build, and while i sometimes will deviate from that, unfortunately at the current time, I don't have the spare funds to buy kits, etc., just for a Group Build.
I still think it's a good idea to have a long timetable for GBs, as it gives me time to decide if I'm going to participate, and what I'm actually going to build.
As regards increasing the participation in GBs, and also solving the difficult problem of finding a Moderator / Leader, perhaps give a version of the system that the Airfix Tribute Forum uses a try ?
At a given time before the GB in question is due to start, in this case I suggest just before the preceding GB is due to begin, a selection of Group Build ideas is put forward for the Forum Members to vote on.
The winning idea is then chosen and added to the GB timetable, AND, the person who suggested it becomes the Group Build moderator / leader.
It sounds somewhat complex, but it's easier to do than to describe . . .
cheers,
Robin.