Well I'm still working on my Master's thesis, so I can't dedicate much time on anything else at the moment.
Luckily for a WWI project, SPWAW already features some WWI equipment that can be used as a basis for a WWI mod - at least a basic one.
The FT-17 tank, for example, was made in France in 1917 but can be found in the Finnish OOB. There's a "female" version with just a co-axial machine gun and a "male" version with a small cannon (SA 18). In fact even the Soviet OOB contains a WWI tank, the T-18 (correct me if I'm wrong). All of these could be used as a basis for tanks of that era. There's also a plethora of trucks and horse-drawn wagons and gun teams that could be used as they are.
Also, the Mosin-Nagant rifle used by early Soviet troops and Finns in the Winter War were made in 1891 and widely used in WWI. Similarly there should be some WWI artillery, which was used by Finns. However, I don't think WWI artillery would differ that much from WWII artillery in the most basic sense, because artillery ranges, munitions etc. are mostly left out of the game as it is. Of course SPA and artillery tactics would be more refined, but the tactics are left to the players - all we'd have to do is to supply the players with artillery of the correct calibre.
Also, the Maxim machine guns should provide us with a basis for MGs. So what else do we need? Early grenades? Easy, SPWAW grenades are pretty ineffective as they are. Bayonets: in the SPWAW mod already. Melee weapons, explosives and flame throwers. too. I really don't think that everything needs to be redone, because SPWAW: WWII already includes such a plethora of equipment that can already be used for a WWI mod.
The infantry models look like WWI soldiers - more so than WWII thanks to their bunched up formation, in fact, and you also have cavalry with rifles and swords. Many thanks to the SPWAW Soviets and their ancient armament!
So basically you now what you need for a very basic mod: small arms, some armour, and most if not all of the artillery should already be in the game. The real challenge comes from creating the OOBs from scratch. Which nations would the mod include and what kind of equipment should they have? Of course this is only a matter of research, but there should be lots of it before the mod can see the light of day.
If nothing else, these weapons and equipment can be used as a comparison.
The most important thing would be to focus on bringing a working mod as early as possible instead of getting stuck on detail after detail. The details will follow, once you get the basic stuff sorted out, and it's quickly
And for that gritty WWI feeling: gas. I wonder if it would be possible to make the napalm warhead look a bit more smokey? ;)
Speaking of smoke: smoke grenades would have to be removed from infantry squads, but as far as I know smoke cannot be removed from the game.
And what about legal matters? There might be copyright issues even though the mod would be completely non-commercial.