I have zero modding abilities or understanding so forgive me if I make ludicrous statements... However, I do have vast miliatary knowledge...thats my focus
Japan: Japan did not take a military part in WW1, however it did hold some PoW's (mostly germans I do believe) So you don't need to add in Japanese units
Trenches: They were rather deep and had dug-outs / sleeping pits in the walls to provide the soldiers with places to sleep (Duh) Some trenches, in Gallipolli, were about 10ft High. How the men fired over, was just a series of steps. This would be a good system to employ (deep trenches) But I dont know how one would go about implementing it, as the AI could be a pain..... I myself would also add in some ladders to go "Over the top" to charge across no-mans land (cos that's how they did it), but in my experience, AI doesnt really climb ladders, or go up stairs all that much.
Also, most of the time, Australia troops did not wear helmets (cos we are so cool) though i do believe on the western front, they did on occasion. They didnt use those weird Pit, Pich, Pitch or whatever helmets that you see the units in Middle East wearing.
I was going to download the initial release, but I shal wait for the expansion.....Would LOVE to see some great missions (a campaign or several would give me an orgasm) Just please! Take your time to get it pristine!
Until then, best of luck
-Matvey The Australian Partisan!