AI that can kill you is easy to make. In fact, games generally make the mistake of stopping there in AI development.
Making AI that can convincingly fail to kill you but will do it sometimes is more difficult.
Also I think it would be nice to have no AI grunts, because you could easily have too many players as it is! Maybe start out with the ability to recruit grunts, but reduce the number of available grunts based on current player load.
I'd love the idea of a simple-graphics (nothing flashy, but 3D) WW2 game with vehicles and stuff. Maybe miniaturized, so your character is a fraction of the complexity of one from Battlefield 1942. The whole point is to present the world and the objects with the minimum system / server resources.
Players who did very well would get promoted, which wouldn't improve their effectiveness but it would give them a command radius. Friendlies within a command radius take less damage from friendly fire and maybe gain some slight bonus.
Equipment loadout should be set up by the player before deployment. He can create several different loadouts so he doesn't have to waste time after dying. Each piece weighs something, and there's a maximum equipment weight. If you get to say half the maximum, you move slower.
Each piece of equipment would give a benefit. Like a radio allows a nearby friendly to communicate on the server radio chat, call in airstrikes and artillery, or call for backup.
Binoculars let you zoom in and see things from farther away.
A pistol is a handy backup if you run out of rifle ammo. Or you could forgoe the rifle altogether to save weight. Or maybe instead of a rifle you want a shotgun, which is shorter range and doesn't work well against body armor.
A heavier gun, like a machine gun, must be braced on the ground but it's really good against infantry.
A bazooka is great against armor. A mortar is even stronger but fires indirectly, which makes it useful only from a distance.
A flamethrower has short range, makes you vulnerable to fuel tank hits, and weighs a lot. But great for clearing buildings.
You could wear body armor, but it takes up weight. A helmet also takes up weight, but helps if someone hits your head and especially against shrapnel and debris from above.
A medical kit can be used to patch up someone to stop bleeding and heal a little, but has limited uses.
Flares can be used to signal or to distract, or to light something up.
Grenades, smoke grenades, poison gas canisters.
Oh and when you call in an airstrike, you're actually calling to players in planes up above. Same with artillery, you had better give them the right coordinates or the players firing the guns will hit the wrong place.
The countryside has towns and stuff, plus natural formations like rivers and cliffs and ridges, which form natural combat zones because they're choke points. Communication lines, radio towers, power plants, factories, bridges, vehicle depots, fuel supplies, etc. would make obvious targets that all sides would want to control. Controlling a site (which would take some time to stabilize) would give you a forward spawn point and whatever resource that facility gives you.
High ranking officers can requisition vehicles and such at any spawn point, and players may need to drive them (and supplies they carry) to some other place for the operation. But these requisitions cost officer points, which renew slowly. The points renew faster if you personally do cool stuff, and if your side has secured appropriate facilities.
Of course, one side will inevitably begin to win. At some point, the losing side will automatically gain the support of some third country that was previously unwilling to enter the fight. At that point, requisition points for them become very cheap, and the winning attackers have to face waves of excellently equipped fighters. If the balance shifts back, the other country stops their support and requisitioning becomes normal again. If they're smart, the losing team will sabotage their requisitioned vehicles so before capture they can be blown up.
But if one side actually wins, then they get a little "victory point" on their account based on how active they were. Say your account is Axis, and you were always at the front doing stuff. You would get the full "victory point". But if you only logged on for a little bit, ot logged on a lot but didn't do anything, you would only get like 0.1 VP. So it wouldn't help to create six dozen accounts on each side, log them in, and have them sit around. These are really just for bragging rights, and for purposes of team rankings. You can tell how important you are to your team, and also how active the members of each team are.
Nor could you switch sides if your side was losing, because your account is stuck with that side. This could also help prevent spying by people who just switch back and forth.
Finally, the best part I think is that it's fun to be on the winning team but it's also fun to be on the losing team. Do you want to try flying a plane but you don't know how? Other people always seem to grab up the planes? Well if your team is losing then there are like a dozen planes just sitting out so grab one and start learning! It creates this feeling os the "last push into the enemy's hardest territory" facing huge waves of tanks and kamikaze jeeps and stuff.