That doesn't sound like what I'm talking about. It doesn't solve how to fit a thousand units on the map when you only have room for 100. The purpose of a leaking spawn system is to flood the map with units (of which 50 is way too low to do that, if the map is "clogged" with 50 units then it's either way too small or the units are way too big).
What I'm talking about is simulating the map as if there were a thousand units that were spawned instead of 100. The other 900 units can't fit on the map so make it look as if they were spawned outside the map. It's impractical to do it literally (try spawning a million units... yes, we mine as well crash the game as it's unplayable at this point).
Whenever a unit is killed, spawn another one. This way the map is always full until the player(s) kill however many units were supposed to be spawned. Better yet, rather than spawning another one, just recycle them (optimization). Whenever a unit is killed, move it to a spawning point and bring it back to life.
This way, it'll look like hundreds or thousands of units are coming at you because the map becomes a "view" (maximum viewing field). The max unit count should be a thousand, but to start with at least 200 (a thousand units may take more optimization tricks than recycling). It could also be dependent on a map, with larger maps (supporting thousands of units) for people with high-end hardware and smaller maps for low-end.
Also with larger maps you can have multiple buildings. So, the player will have an incentive to move around more (besides avoiding zombies).
Another interesting mechanic would be if more zombies were spawned near a player that is near the edge of the map. Perhaps it could be used somehow to prevent players from getting to the edge of the map (so you can't walk off the map, or you have to use an "invisible wall", zombies preventing you from walking off the map is better IMO).
I don't think zombies should upgrade with each level, but if they do then the player needs to be able to upgrade as well. This will make balancing the game harder (but it might make it more fun). I think upgrades should be left out at least until the game is more complete.
EDIT: Also, rather than having spawning points, zombies should be spawned at the edge of maps as if they were walking in from a random direction. This way when it becomes multiplayer, players can't farm spots and everyone will have a chance... I should probably write up a document or something because there are a lot more details that I could go into besides the above.
