Lots of great ideas, guys! Forgive me if my ideas are just repeats.......
1) A 'what-if' build, including Luft '46, paper panzers, etc
2) Something with animals such as horse riders, mine detecting dogs and their handlers...............
3) Something from the recent Middle Eastern troubles (not sure if that's moral as it's still going on.........is it?)
4) Trains or something on rails such as armoured wagons, etc
5) Spanish Civil War - lots of possibilities for aircraft and armour
6) WW1 scene, tank, figure, plane, ship, sub
7) Korean War - armour/aircraft
8) Vietnam - armour
9) Battle of the Atlantic - ships, subs, recon-aircraft, etc
10) captured vehicles in service with prior enemies (e.g. Russian Tiger I, German T-34) - endless possibilities.
11) Improvised fighting vehicles - like the pick-ups with massive anti-aircraft guns in the back
BTW, I like the strange aircraft build some suggested.