Interesting topic. Here are my thoughts on it.
The strategic impact of tanks and planes on WW I battlefields were very limited. Tactically, yes they had an impact, but at this level they'd not even warrant a unit. Tanks were not really organized above brigade level for example and were parceled out to infantry units regardless (a practice France continued in WW II even though the French had more and stronger tanks than Germany in summer of 1940).
You could have armor available as a research option and allow units to "upgrade" to it (say 2 levels max) giving them a soft attack bonus (strictly offensive units here not used on defense by allies).
Also how about an "entrenchment" research option? This is how I was thinking of simulating the trench warfare aspects of WW I (I to was thinking of using SC2 for WW I, just never got around to it yet). I'd have the Germans start with entrenchment 1 (they had better trench lines throughout the war than French and British) and this ability simply increases soft defense by 1 per level.
Planes too can be researched and added to units as a way of showing their original mission--reconnaissance. So an army can increase its spotting range (set the defaults to 1 for spotting with ability to increase to 2 or 3). Planes were limited in payload and range (and numbers compared to WW II) so they really have no strategic impact as strategic bombers or tactical bombers.
Gas attacks would increase soft attacks to help break the lines but be expensive and only go to maximum of 2 levels.
I'd definitely keep the action points low for the corps and armies (meaning no motorization research). Everything was foot or horse powered really.
HQs with low command ratings (the communications and lack of transport) and maybe 1 less unit per HQ than the standard default (Prussian General staff was excellent and pretty muched same command system Nazis used in WWII). So I'd make "good" HQs in the 5-7 range tops.
My first post on forum btw, sorry for all the verbage.
-Mark