Wait.
I do not think that the game can handle a scale that big.
1000 Enemies? Nope.
Serious Sam games can spawn lots of enemies and there was that "Starship Troopes" game that was released in 2005, that could spawn about 300 at the same time, but i have not seen anything like 1000 enemies in an FPS.
Hitman: Blood Money can spawn crowds of people, but these are just NPCs who just walk around or pre-arranged paths.
So we better leave this 1000 enemies thing and make it to spawn more ZEDs in total that at the same time. FakedPlayers Mutator can Spawn 999 ZEDS.
What i think about is KFO Maps, of reasonable size. Outdoors map can be bigger, imagine Farm and MountainPass for example.
These maps will have vehicles, not military, but modified civilian vehicles with improvised armour that will get damaged, and players will have to complete objectives, or they can play a normal KF game. Military vehicles can be added as part of objectives. Find fuel, get to the survivors, get them to safety. Something like that.
ZEDs will come in greater numbers. Double or triple the amount for each player.
This may also be the basis for a new Engineer class as well as LMG additions.
Obviously this is not an essential suggestion, but i would not mind a couple of such maps, or even more.
An "Arena" type mode for PvP wouldn't be essential, but many people would like it.
As long as the core game remains the same, i do not mind.