Let’s do this by a combination of what makes sense thematically and mechanically.
Aircraft Carrier: T: While some experiments existed it feels way too early to see these on the board. M: Works well, obvious mechanics taken from WWII A&As
Airship/Zeppelin: I think a reasonable choice. I can see potential mechanics. Perhaps boosts naval ships?
Armored car: T: Armored cars were definitely used a lot in that war. These weapons were very useful only in very specific circumstances - with roads. They are powerful on roads, useless outside of roads. M: More problematic. I can’t see how to model that in game. A unit that moves 2 spaces might be nice. Kind of Mech Inf for WWI?
Armored train: T: Makes me think of the Russian Revolution more than WWI. But yeah. Same era. M: How do you represent being restricted to rails in this game?
Askari: T: The game has never used national origin to affect unit type. The closest we’ve ever seen is ANZAC being its own power, but that was a difference in power, not unit type. M: An infantry built on colonies? Do we need a different unit for that as opposed to just allowing colony builds? What could possibly make them different.
Bomber: T: A little premature. While there were bombers, they were much closer in structure and performance to contemporary fighters than in WW2. Strat bombing was in its infancy. M: I don’t see any room mechanically.
Cavalry: T: Cavalry was on its way out, really were used as dragoon more than anything else. Yet, I still like the idea. Calvary was definitely used. M: Making cavalry move 2 makes little sense. So, then, what do they do? Attacking a higher number makes no sense. Their increased mobility would be hard to model, except by moving 2 spaces, but I dislike that idea.
Destroyer: An obvious choice, pretty much AA40 rules, but I think leaving them out was deliberate. Submarines are stronger without them, and I think this was on purpose.
Elite Infantry: T: Unit experience really isn’t modeled in A&A ever. M: Infantry that attack higher? Eh, bland.
Fortress: T: Fortifications were a big part of WWI. It feels a little too operational-scale for a strategic-level game though. M: A static unit built on location with high defense perhaps? I don’t like it. Would slow down the game, I don’t see too much benefit.
General: T: NO. Strategical level game. The generals are part of your infantry units, just like machine guns and mortars. M: What would they even do?
Heavy Artillery: T: I think the full spectrum of artillery weights are represented by the units. M: What would they do? Boost two infantry?
Railgun: I think a neat idea but one that would be too hard to do mechanics for. How do you represent what makes them different from artillery on a game of this scale?
Sea Mine: T: Already covered. M: You can move your sea mines? That’s… silly. When has anyone ever mined a harbor that’s not their own? A mine layer would make more sense, but only just a little. I don’t like it.
Antiair: T: Sure, yeah, makes total sense. M: The mechanics write themselves, from AA40 or other possible mechanics. Does the game need them, though? Air power is not nearly as powerful as it is in WWII games, and I think the decision to counter aircraft with aircraft exclusively was a conscious one on Larry’s part.
Flying boat: T: Were these really a major part of WWI? I don’t think so. M: I can’t think of what these would do.
Of these, Armored cars, Cavalry, and Destroyers are the only ones I would entertain. Perhaps you’d like to flesh out your ideas more, instead of just listing equipment used in WWI. Perhaps we’ll find an interesting mechanical space to make a new unit.