(You indicated WW
II, the first time. Slip of the fingers, of course...)
When it comes to Flight Sims, there's loads of previous WWI-era (or 'includes WWI-era) flight-sims, but of course "better graphics" would come to the fore. I think an extra dimension might include additional work to assemble the 'crates' shipped over from blighty (or wherever, or ferrying them to the forward airfields from some rearward staging spot or even the factories), and the fact that you might be able to survive crashes (or parachuting, if you're not in the RFC) into No Man's Land and neither die-outright nor 'automagically' find yourself back at your airfield. (Of course,
in extremis, this could turn out to be an "escape from POW camp" sim.
)
Similarly, naval sims are pretty much out there. Perhaps something with a bit more AI for the subordinates beneath you (assuming you're in command) or the commanders above you (if you're supposed to be a midshipman, gunner, whatever, instead of the traditional single-/multi-ship command position.
The infantry experience
does seem unpromising, I'll admit. I've played WW1 trench games in table-top wargaming format, and it made is
so depressed. (Figures placed in trenches, one player decides to charge, loses 90% or more of their men. If the other player then decides to attack he then loses 90% or more of the men, even against the aforementioned
depleted forces. Any tanks, planes and other vehicles that take part generally end up breaking down, crashing, hit by friendly fire, or something. Apparently quite realistic!)
Though i suppose it could be done (with some background-AI fudging so that you never
quite get "a blighty one", and always have a decent chance of getting away from the gas, shelling, etc, while play progresses) insofar as the decisions you make and the action that you
do see sends you up the Field Commission-ladder. Not that you'd ever get to be sitting in Haigh's shoes (or mightn't you?) directing the whole battle-front (and, some would say, you couldn't do any worse at doing so!) but you
could affect the topology of the front. Of course, when you're "just" controlling the maps in front of you you're not guaranteed to have all the right information, and the closer to the front you are (to get better communications, both ways) the more danger you're in from sudden changes in the front lines and whatever ranged/aerial attacks the enemy might send against you. Or, indeed, underground!
Because I
really like your sapper-based ideas. And I can't think of much to add to this, save for the possibility of a similar "overview" mode where you're not running them in FPS-mode, but directing efforts (perhaps in a time-compressed mode?). Again, with inaccuracies and deviations from reality...
(Hmm, Ace Of Spades... might have a look at that, later.)