Because there will be so little room when such a thing is implemented (if it is implemented), I think that the custom game menu should be multiple screens. One way would be to use tabs (eg: one tab would be the player factions, teams, etc, another would be the map, with a full sized map preview, the tileset, fog of war, etc, another could be game types, and so on. This would also make further expansion easier) alternatively, a "next" and "back" system would be virtually the same as tabs, though instead of having tabs on the top or bottom, there would be a simpler next and back button that would take you through each of the submenus.
That'd, of course, be a thing for Glest 4, of course, in the distant future.