Quote: "I'd be interested in hearing any ideas on why the player would stay on the grounds instead of just running away."
I can think of four types of reasons for staying
A. Something physically stopping you: aliens surrounding the grounds or you somehow become trapped inside the house.
B. An emotional reason for not leaving: someone else is trapped or injured and you don't want to leave them to die.
C. Duty: you can't leave because YOU must stop them.
D. Self sacrifice or hopelessness: there is no escape or you are infected and will die/turn into an alien and cannot be cured. You decide to go out guns blazing!
Let's look in a bit more detail at these options.
A: Trapped
A physical barrier is the most obvious, but what would that be?
An impenetrable ring of aliens suddenly appearing is hard to explain and trees or power-lines falling down and blocking your path is very coincidental. Or maybe the reason is simply that he doesn't want to risk running from the aliens and thinks it is safer inside.
B: The Protector
Maybe there was someone else in the car with you when it crashed. They are badly injured and you carry them to the farmhouse.
Alternatively you could discover someone at the farmhouse who is still alive but is badly injured. In both cases some extra modelling and animation is required, maybe even voice acting.
C: Duty
I can't think of how we could use the "duty" reason because it suggests hunting down the aliens and exterminating them, which involves too much moving around.
D: Hopelessness
There is no more detail to add, you simply accept that you're going to die and try to kill as many as you can.
- Summary -
Personally I think the simple fact that there are loads of aliens outside is good enough reason to stay but maybe what I have written will spark someone else's imagination.
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Now what about those aliens?
Why are they attacking this farm?
Farms are quiet and isolated, so the aliens must be keeping a low profile.
Why would they do that? They obviously like killing people so why not attack a city?
They obviously aren't capable of taking over a city.
So they are either here by accident or they are preparing for a larger attack.
If they are here by accident why would they attack humans? Let's leave that one for now and assume they are planning a bigger attack.
So, they are attacking the humans to improve in some way. Maybe they lay eggs in humans or infect humans so that they turn into aliens, both effective ways of raising their numbers (and both champion the "hopelessness" reason for staying). Humans could simply be food for the aliens or their technology requires living organs to power it or some other creepy thing like that.
- Summary -
I like the idea of an infection that slowly turns the protagonist into an alien. It offers a few game mechanics: there could be a bar that tracks how far you are into the transformation, receiving damage from the aliens could speed up the process. There could also be an eerie option of suicide which would score you bonus points if you killed yourself before you completely turned into an alien.
Hope this helps.
[edit]
BTW I am attempting to make a bucket, it might take me a while as I am learning Blender as I go.
"With games, we create these elaborate worlds in our minds, and the computer is there to do the bookkeeping." - Will Wright