Large expansions in a WW2 setting, definitely. That's what I think would work best, like RS worked. Italy or Africa preferrably, but Barbarossa '41 or Normandy would be fine, too.
Perhaps make it patially some kind of community project: you tell us the theme and the deadline. Those who contribute content (static meshes, weapons, uniforms, maps) get the game for free or, depending on how much they contributed, will get a small (and I really mean a more symbolic) reward.
For example, you say Italy'43, deadline: one year from now on, minimum requirements for static meshes: 3 LOD's, diffuse, specular and normal map, trees must be animated. We need trees, buildings (preferrably modular) and props like...
Also don't underestimate the smaller goodies, like customizable uniforms. As you can see in the forums, there is a huge demand to customize the character. I have seen that in other games years ago, where clans liked to dress in 'their' uniforms. It would be great if that could be integrated into all game modes, if necessary seperate the uniform mod part from the mod that plays with other game settings.