You may be right. The fact that nobody explored the other mentioned ideas may be not because of the lack of time, but because of our collective gut-feeling. Then, indeed, do not include them individually in the 10 top variants listing, but reserve for them
10. Other (cliche, unbelievable, too hard to implement without the "authors are lazy" feeling, other)
'Revenge for Killing their Scouts' seems completely unrealistic to me.
I agree. Civilized aliens would just come and resolve the misunderstanding or make us punish the wrongdoers (e.g. CIA
). And warmongering aliens do not need pretexts.
Invasion is never about revenge. The resources and time can never be justified by retribution.
I think you quietly assume Aliens live under democracy/oligarchy/etc. What if they live under single-person tyranny? Then there is a plenty of examples of wars fought merely for personal revenge (even if WWI was actually not). Of course if a mad tyrant rules aliens, again his particular reasons are not that important and their believability even less so, which makes it quite cheap.
I will attempt to explain the rationale behind invasion, in terms of economics and strategy:
Nice reading. I have a warm feeling our mainstream variants are in accordance with your analysis.
To commit to an invasion, it must be possible to continue the invasion as long as necessary, that is 5 to 10 times the expected time frame. That is you must have the resources and units to sustain the warfare.
Unless no resistance was expected and still the goal of the expedition is
very important and urgent. (We do not want a total war so that our beloved squad combat actually has any importance.)
The motivation for the invasion must be rational enough that all troops employed will agree or understand it.
Again a lot of assumptions about alien psychology (I like that, I dislike the cheap explanations "do not try to understand; aliens are just very alien"). But then you exhibit some strange assumptions about the lack of propaganda, chauvinism, brain washing, wishful thinking, rationalizing in the face of brutality and trauma, etc. in the alien army. Is it so different from our armies?
Lastly, the whole Mumbai scenario really needs to get thrown out. Its no way to start a game plot. You don't start a plot straight off with a climax, because the plot intensity will be going down hill from then onwards until its ready to start building up again.
I'm with BTAxis here. Yes, starting with nobody certain about ufos, the need to convince UN, peacemongering, etc. makes for a very interesting plot, but not quite for this kind of game. I feel the openning scene where the aliens are shown as very powerful and very cruel is very good. Then you start the game, prepare your squad for the worst, sit on your hands for a day, for the second day, for the third day, a UFO appears, but nobody is able to chase it down, the fourth day, the fifth day...
Then the siren rings and you begin experiencing the Mumbai horror on your own skin, step by step, consious all the way that what you see is a fraction of the Mumbai forces and still you return to base with half you squad dead or wounded, or most civilians killed to the outrage of your sponsors. And the aliens start bring better weapons and soldier than the onces in Mumbai and their number increases.