I've been reading through your development wiki, and saw that alien activity will be based on interest (starting low, getting higher at speeds determined by difficulty level, etc.). The list you have looks good, and I know there's already a long list of things to be developed, but I had an idea to add to the list of alien activities: propaganda.
This could be something like mind control, or even just voluntary support of the aliens by humans, in the form of human barriers between your troops and alien bases, or front line militia that the aliens use instead of their own troops. Even spies (could even be aliens disguised as humans?) infiltrating your base and sabotaging something. It would definitely add to the "paranoia" and widespread panic feel that would certainly exist if ever aliens did try to destroy/take control of the planet. If nothing else, current events have shown us that (no matter what side of this conflict you're on) some people will follow leaders, no matter how destructive or crazy the mission seems to other people (whether it's for money, religion, or nationalism). I bet at least some humans would embrace/follow alien leadership, but it's not essential to your story, just would be a neat addition in my opinion.