Will you guys add some cavalry classes? Two would be enough, namely some type of dragoon/sabre cavalry and lancer cavalry. Both are equiped with carbines. Historically they were still used as a very mobile infantry force. Especially in conquest they would come in handy, to quickly take over a certain position. Heck, sometimes a good old charge may be on its place. It would also allow for some fun scripted events or fun rounds. As there are no tanks in this mod, and I don't know to which degree it is possible to add them, there is a lack of rapid moving mobile forces.
Please add some more regiments. For now there seem to be only some standard units, instead of a wider unit variation. It would give the mod some more colour.
I don't know if you're solely focusing on the western front or not, but if so, please add the UK and Belgium. If not, add Russia, Ottoman Empire, Austria-Hungary, Italy,... the big factions. If you take on the eastern front, and Ottomans, then I return to my first point. Cavalry was very much still used in these more open pitched battles. Cavalry should definitely be included.
Adding more weapons and weapon types would again add more flavour to the game. The French and Germans used more weapons than the Gewehr 88 and the Lebel rifles.
Maybe less practical to add, but having the option to switch between new and old uniforms would make for some nice scripted events, that aim to portray a specific part of the war.
Somehow that Parabellum mod succeeded in adding planes and tanks. If you could ask them how they did it, you can maybe add it too?
Both heavy and light machine guns aren't that powerful. I think you ought to buff them a bit.
Those are my suggestions. Do with them, as you will.