Sorry, it was me who used the term 'engine', so let me clarify:
In general, 'engine' can have a multitude of meanings.
In this particular case I meant "the code the executes the campaign scripts".
In UFO:AI 'engine' usually refers to the part of the code we inherited from Quake.
btw I don't think the renderer is friend/foe-related. At least it shouldn't. But you'll never know
However, before talking about code changes, we should think about the differences in gameplay. What immediately comes to my mind is
- aliens can't build their bases like humans do
- aliens don't intercept human aircraft
- aliens don't *wait* for missions, they initiate them
- would aliens research human technologies ? A little bit maybe.
- do aliens have a budget ??
- and so on.
The more I think about it, I have to say it could be a fascinating game. The changes to the battlescape code will be the minor part. But if done right, we'll need additional code for some 70% of what we are doing outside battlescape.
With the current dev team, we could achieve that by 2020
But things sometimes change rapidly. With half a dozen of additional devs, it could as well be 2011. So in order to atract those 6 devs, you should start with a good gameplay concept.
I still think it might be a *great* idea