Nice! I really like ww1 infantry. I personally feel it bridges the gap between the Napoleanic style of warfare and ww2. As for combat bonuses I've come up with some suggestions.
Great War Infantry
17

1

130

Unlocked At: Journalism
Requires: Rifling
Abilities: +10% vs. Gun Units
Shock Infantry
19

1

135

Unlocked At: Plastics
Requires: Assembly Line
Abilities: Doesn't Receive Defensive Bonuses
+25% Attack vs. Machine Gun
+25% Attack vs. Artillery
+15% Attack vs. Gun Units
Flame Trooper
15

1

120

Unlocked At: Pneumatics
Requires: Refining, Natural Gas, or Oil
Abilities: 2 First Strikes
50% Chance to Withdraw from Combat
Maximum 60% Collateral Damage (5 units)
50% Hills Attack
Can Bombard City Defenses (-8%/Turn)
Landship
20

1

180

Unlocked At: Explosives
Requires: Oil, and Iron
Abilities: Doesn't Receive Defensive Bonuses
Cannot Move Over Hills, or Wetlands
-20% Against Artillery
+25% vs Gun Units
A Few Other Changes
- Normal Infantry is moved to Pharmaceuticals(ww2 infantry were much better supplied with drugs and medicine than those of ww1) but otherwise stays the same.
- Tanks are moved to electronics (they need electric motors for their turrets to swivel ) and stay the same.
- The Machine Gun now has 100%
vs cavalry units and gets a ww1 reskin.
- The Longbowman's maximum collateral damage is reduced to 40% (because the grenadier should become a skirmish unit with the addition of the Flame Trooper as one).
- The Grenadier is moved back to Logistics, has its
reduced to 8, and receives 2 first strikes, a 50% withdrawal chance, the Longbowman's 50% collateral damage but with 4 units max, 50% hills attack, 50% attack vs. the Musketman, and -5% city bombardment per turn.
- Also the Grenadier should get that much needed 16th century reskin when they actually used grenades. I mentioned it on an earlier post
(perhaps you could make that happen).
And finally a new unique promotion that absolutely screams ww1...
Toxic Gas Promotion
Unlocked At: Chemistry
Requires: nothing
Leads To: nothing
Available To: Siege Units, and Air Units
Effects: 100% maximum collateral damage.
Can Bomb City Defenses an extra -5% per turn.
Incurrs a diplomatic penalty each time the unit attacks/air strikes or bombards/air bombs defenses.
Note: The mobile SAM will have to be moved back to the gun unit category because it doesn't really make much sense for it to be able to get this promotion. You know like shooting poisonous missiles at planes or drones in the air. It would make sense for the other units in the siege and air groups, and the rams and other stuff will be long gone by the time this promotion becomes available (a poisonous ram would be very bizarre). The SAM would only make sense if it were a surface to surface missile battery which of course its not. SAM meaning surface to air missile and toxic gas is most effective on the ground (not the air were the pilot is wearing an oxygen mask in a pressurized cockpit or a drone that doesn't have lungs to begin with).