Just thinking about these to add some randomness to the game.
Assume a maximum 20 rounds in a game; that is 4 per year staring Autumn 1914 ending May 1919. At the end of the 20th round winner is calculated on income, victory cities or whatever.
Assume turn order is 1 - All Central Powers play, 2 - All Allies play
Each side has its own deck of 52 cars. Start with a hand of 6, each side picks up a new card at the beginning of a round.
Each side can play one card on its turn; then the other side can play one from its own deck, possible a card to counter the one just played.
Possible card types:
1. tactical: covering items such as poison gas, smoke shells, undermining etc. So there’d be a number covering each of these, and a (probably smaller) number covering the counter-measure such as Gas shells - Gas masks.
2. political: introducing events such as revolts and mutinies. Some of these might move a particular country closer to revolution.
3. weather: effecting individual battles; for example “fog” would negate the defensive advantages of the enemy for one battle, not for each battle in the turn.
4. economic: could give a country an IPC boost, reduce the cost of a certain unit for a turn, or bring bonus infantry recruits from a patriotic rush.
I suppose techs could be integrated as part of this, but I’d rather use the old “pay to develop system”.
I prefer techs which can be represented by a physical unit, so the above would cover the more abstract methods of war.