Like ossel, I’m for 6 techs, a la A&A Classic.
Instead of a reroll for Chemical warfare, how about the following to simulate gas masks:
If Chemical Warfare is successful against you, halve the number of infantry immobilized, rounded down (4 becomes 2, 3 becomes 1, 2 becomes 1, 1 becomes 0).
For Radio Communication:
For each artillery unit of an ally in the same territory, one infantry or tank is promoted to the with artillery support box. Allied artillery does not participate in battle.
Instead of Advanced Aeronautics:
Strategic Bombing - Fighters within 2 spaces of an enemy capital may perform a strategic bombing raid (Thus Germany can hit Paris or London from Belgium) instead of engaging in combat. Each fighter in the raid rolls one die, deducting 1 IPC from the attacked power’s treasury for each 1 or 2 rolled (the airborne version of USW).
For Undersea Warfare (Anti-Submarine Warfare):
Your Cruisers prevent enemy subs from submerging in battle or moving through a sea zone you occupy at a 1:1 ratio.
Since the first aircraft carrier wasn’t operational until October 1918, change Naval Aviation to:
Your fighters may be assigned to a naval combat in a sea zone adjacent to the territory they occupy. Fighters in naval combat behave in the same way as in land combat: establish air supremacy, strafe enemy ships, boost surface ships’ combat values by 1.
Replace Tanks with AA Guns, which were pretty much born in WWI, although primitive:
AA Guns - If enemy aircraft are present in a battle, roll one die before the battle for air supremacy begins. On a roll of 1, an enemy fighter is destroyed. If an enemy is conducting a strategic bombing raid, roll one die before the enemy fighters roll to determine IPC damage. On a roll of 1, an enemy fighter is destroyed. - Could be changed to roll one die per artillery unit, but getting the altitude right was tough in WWI.