Just in the round...
What´s about accoustic weapons that stun the enemys with light damage like those grenades?
A heavy weapon for vehicles with a wide range (20-30) and a conical area of effect. The germans developed some engines on this base in wwII but without success. Why not 35 years later? As tech i would set it after Buzzard and Corsair, 2 techs who works with accoustic in widest mind.
In addition for balancing, the aliens should use "pods of death", spraying toxical gases around the Battlefield who kills everyone earthborn. Like fire wanders, this gas should wanders through map.
Without a good armor, the gas will potential deadly, but I think its in the techtree for later UFO´s, all above medium, so it will hurt a soldier slightly per round inside the gas (not the run through).
Further... what´s about the use of little drones, smaller and unarmed like the tanks, with levitation and a lightsource.
Ok. My english isn´t perfect, just laugh about it or use your brain for the content