Be it in a infection map, or possibly in firefight, it would be possible (and totally cool) to add flood spawn points.
Spawn points for AI in video-games can often get really complicated, but I believe that it would be easier with flood, since they know where the player is anyway. Then its only a question of how many will attack at once.
You could add individual spawn points with a max of lets say, 3-4, and one large hill marker in which flood can land their airborne flood pods. All types of flood can spawn at each location, controlled by gametype options.
The spawn points will be placeable in a part of the map that is inaccessible to the building of forge structures, or simply just on maps that dont come with structural parts. Or they could simply spawn at the nearest open area if someone places their spawn-point inside a box.
Imagine playing infection, in a defend-the-castle like map, where the dead players are backed by increasing numbers of flood!