We have this excellent thread going for individual kit ideas.
This thread is to present monthly theme ideas.
I will say off the bat I like themes and ideas that augment existing plastic kits. I.E. Command Groups, heavy weapons, etc., for existing plastics. STLs like that will enable one to actually field a fully playable army/force.
Casualties and Zombies has already been proposed. This is a fantastic idea. Casualties are geat for casualty/fatigue/pin markers. Bonus if WGA includes a printable base for the casualties that has a dial or at least holds a dice to record numbers.
And of course Zombies make every game and setting better.
Command and Control is another already proposed theme that would be great. Officers, NCOs, Color Bearers, medics, RTOs, etc.