Idea is to have a running timer the second a city starts being terrorized. The longer you take to get there the more points you'll lose at the end of the month. Penalty proportional to the delay
As an additional difficulty set a random chance the terror site disappears every hour after say the first 4-5. The aliens have finished killing/abducting all the civilians and leave before you could arrive. Increase the penalty/unhappiness/chance of surrender of the country targeted if you failed to even get there
I would have no complaint about having something like this as an advanced
option - turned OFF by default of course.
These two things are to prevent the 'wait until sunrise' cheat so many players use by turning that strategy into a cost/trade-off
This is a fairly standard exploit. I'd say that it isn't the intent of the game to try to randomly
prevent you from handling a terror mission entirely - the point was to get you to handle terror missions
immediately. Unlike investigating crash sites this was meant to introduce the
possibility that the only way you can reach it in time is to risk arriving at night. Not EVERY terror mission was going to take place at night but it was going to be a possibility which early in the game was difficult to avoid (if you didn't just abandon the attempt and eat the high penalty to your score).
Now, if you want to program out that exploit I'd say what needs to happen is this:
First, you CANNOT target the terror site with an interceptor or other craft that can't carry troops. If you try then you should get a popup that tells you so: "Hey dummy, there's nothing for you to shoot down here. You need to send troops - and the clock is ticking!"
Second, the VERY FIRST craft that you do legitimately target the terror site with is the ONLY craft that you can target the terror site with. If you then give that craft an alternate order, sending it home, giving it a new target, etc. the terror site immediately disappears. This would be the way you want to enforce that clock. Once you target the site the clock is stopped and the terror site will wait for your troops
imminent arrival (whenever that will be) but if you then VOID that by changing your mind... you lose. Thus you CAN still delay sending troops instantly and TRY to time it for daylight, but once you do start for it you're committed and can't keep it live indefinitely.
Third, you can still theoretically exploit this just a LITTLE bit. For example, a possibility I thought of is this: A terror site pops up. You target it with a skyranger that will be coming from the other side of the globe. That stops the timer, but you can still get there EARLIER with a different troop ship, either something that travels faster or that is launched from a closer base to get there first. The terror site will be handled by the first troops to actually arrive, but the TIMER is stopped by the first craft to target it and the arrival of that first craft then acts as the "final countdown" to the site disappearing if not reached sooner.