Relic has done WW2 twice now and The Great War mod shows a lot of promise for CoH in WW1, a friend of mine even made an edit of that mod focusing more on the trench warfare aspect of the war in 1915-1917 instead of the more mobile and open warfare of 1918.
Here's a link to the group for anyone who is interested in seeing what exactly I'm talking about:
https://steamcommunity.com/groups/TGWWVPMedit
Anyhow, my point is that I think the PC FPS and RTS market is oversaturated by WW2 and Modern games and there are few set in WW1 so there's a lot to experiment with and develop, plus having some recent popular games such as Battlefield 1, Iron Harvest (sort of) and Verdun/Tannenberg draw a lot of attention to WW1 there seems to be a huge lack of WW1 RTS games, not to mention that lack of good such, most are either mods or incomplete.
How I imagine it would work out is as usual as CoH has been, you start a match as either 4 of the Entente Armies (France, British Empire, Italy and Russian Empire) or 4 of the Central Power Armies (German Empire, Austro-Hungarian Empire, Ottoman Empire and Bulgaria) plus some minor countries such as Canada and the ANZACs appearing in doctrines for the British while perhaps the US could be featured as foreign reinforcement units for the French as it was planned in the TGW mod.
Anyhow, the match could go in 2 of these ways: Either mobile warfare of 1918 (or Eastern Front) ala vanilla CoH but set in WW1 or full on Trench Warfare.
How you would achieve that is by having the player choose between either teching up with his fuel or deciding to dig in by making trenches cost fuel as well.
Pillboxes would be constructed by engineer units while weapon teams such as Machine Guns, Trench Mortars, Field guns and Howitzers would play an even bigger part in the game than before as they will be the main tools you will use as to hold and attack trench lines.
To go in a bit more of a detail in these weapon systems, you can probably already guess the role of the Machine Gun so I won't go into it, Trench Mortars would basically act as light and short ranged indirect fire artillery meant to weaken the enemy in their trenches, Field Guns would basically act as AT guns but also as bunker buster weapons that could also be loaded with shrapnel for use against infantry in the open as well, they could either be pushed slowly or towed by a truck. Howitzers would be heavier indirect fire artillery capable of firing a barrage of rounds in a target area, smoke, gas and creeping barrages from specific doctrines. To top it off I would also suggest these weapon teams being able to dig themselves in.
Heavier guns such as the Big Bertha could perhaps be only towed or built by engineer units or just featured as heavy off-map artillery.
Some maps could also perhaps feature "No Man's Land" a territory between trenches that belongs to no one, meaning it could only contain barbedwire, mines and sandbags as well as shell craters, small pieces of land on the map borders would connect the territories that are separated by the No Man's Land.
Vehicles such as Staff Cars, Trucks, Armored Cars, Tanks and perhaps even Cavalry would play a huge role in the game as well, they will respectively have their own roles such as transporting officer units around, transport guns and infantry around the front where they are needed, feel in gaps in the line and help assault enemy trenches, Cavalry as in the real world would be useless charging MGs with but perhaps could be used in great effect in the early trench-less game phase or if the game doesn't go the trench route.
All in all I believe that the concept of CoH in WW1 that The Great War mod and soon to be Iron Harvest are presenting is a good motivator to go for something new and not as experienced, CoH has the needed systems in the form of cover (I forgot to mention that there would be a 4th type of cover, Trench Cover), terrain destruction and so forth in order to portray a believable WW1 game scenario, not to mention the many stories which can be told such as those of the ANZACs at Gallipoli and Africa, the Canadians at Passchendaele, the French at Verdun, the German offensive of 1918, the Bulgarians on the Balkans, the British at the Somme, the US at the Meuse-Ardennes Offensive and so forth.
Thanks for reading, cheers and have a nice day.