A wee bit off topic but anyway ...
I'm going to say that it's near impossible to make a really meaningful war game out of a FPS. Don't get me wrong I love FPS and think you can do a lot out of them but I just don't think you can make it work such subject matter.
First of all the got the massive skill gaps of FPS players which can really affect the mechanics and break immersion. A good example of this would be Telltale's Walking Dead and (early) Far Cry 3. In both of these games you're meant to be an average person with no skills in weaponry, however when a decent shooter player performs the parts that involve shooting the characters suddenly become walking torrents of death. The only way to compensate for this is to weaken shooter gameplay by artificially inflating shooting difficulty and spawning less enemies. Which, in my opinion, defeats the purpose of making it a FPS in the first place.
I would personally say that FPS tend to heavily desensitise the player to violence in the
gameworld. Shooting is satisfying and largely rewarding. It would be extremely hard to come up with a mechanic that's not annoying and punishing to make sure that shooter player doesn't feel that way.
It seems like something that would be covered better by some sort of "survival-adventure" based around a small tightly knit group of soldiers who slowly develop stronger relationships. It's main "gimmick" could be the squad mates or the player character(s) dying in completely inane and unsatisfying ways.
Now I know you're all going to bring up Spec Ops: The Line as an example of a shooter handling war themes well. I won't disagree with that. However Spec Ops was also a deconstruction of the modern shooter as well. To try and make a game purely about warfare and not about the difference between games' interpretation of warfare and real terrible warfare is something I just can't see being done by a FPS.
WW2 games where there is no Axis campaign
I think it's a failure of history that all Germans during WWII are all considered amoral monsters. There's a big difference between the basic Wehrmacht and the SS Death Heads. Yes there were atrocities committed by both sides but the fact that the brunt of the german army (and all armies at the time) were barely trained conscripts forced into service is something a lot of people fail to consider.
Note that I'm not justifying any action taken by Nazi forces during WWII rather that there were people serving in the army and not just monsters.