A few suggestions for further growth:
Lurk Triggering:
If a player is in within x metres of a predefined location for more than y minutes it triggers a strike.
You could triggers in places people lurk - like sniper hill in electro - and set the timer for long enough for a spawnkilling scumbag to get settled in. I'd personally love to see a strike come in and watch 3-4 campers running like hell off that hill.
Would be good to set the sirens to sound in the nearest town of course, so I guess you'd need to have two locations defined for each trigger (siren & strike).
Less Tick:
Make the ordinance arrive in clumps and more randomly. Right now it's kind of like a ticking clock: whisle/boom/whistle/boom Might even be nice to choose how many crews are actually firing, so crews = 5 means 5 sources with a random delay between firing for each crew/shot (if that even makes sense!).
Chemical attack:
The ordinance isn't HE, it's green smoke. The green smoke spawns x zombies. These zombies are annoyed.
Zombie Aggro:
The air raid sirens attract zombies in the same way firing guns does.
As you can probably tell, I don't do much scripting myself - I'm not even sure it's possible to spawn zombies outside of the normal loop.
EDIT: Oh yes, It'd be nice to be able to switch off the heli/ai & also have a seperate config file to keep things tidy. I'm rambling. I'll shut up