I don't know. A random "draft" might be kind of fun and probably very balancing given a large enough participation size. Not only would it spread out the number of members, but more importantly, spread out the number of "active" members. As Kenny mentioned, it's these dedicated individuals who usually earn large numbers of points for their teams.

In fact, any balancing system, if done well, should probably take that into account. A team with 5 members who participate in the War like crazy each year will outperform a team with 10 members who participate only one or two weeks a year. I wonder if there is a good way to measure this "density" for each person.

Elysia's idea essentially says that measuring such "density" doesn't matter because it will be balanced naturally.