on more advanced missions, aliens could ambush soldiers right on the battlescape like this:
2-3 aliens hide in buildings at windows aiming at a certain bottleneck on the map.
1 or 2 aliens could serve as lure for the others, giving them easy firefight and retreating to a point where soldiers need to follow them right through the bottleneck (if they whant to chase them fast enough to catch up).
or let them use "other" equipment, not just "better" stuff, like weak anti-personal mines or more aliens with area effect weapons (like stun grenades, not only alien grenades).
in a mission where you shoot down a ufo the primary target for the aliens should be to protect the crash site till the engineers repaired the ufo to a point where it can fly short distances again.
having this in mind the aliens should be more the camping/ambushing race, giving every soldier a hard time wich tries to rush the map.
they should do this atleast after mission 10 - 20 when they learned that the xenonauts raid every crashsite of them.
I can report this too, thought it would be normal.