XCOM lore showed us there will be governments, who will completely succumb to the Alien Agenda. Look at whole continents and islands today becoming totalitarian in a way that suggests that aliens are now mass-texting the governments, what dictates to make, what tyrannical mandates to do. So any country lost to the aliens could and should organize raids on XCOM bases.
Even governments that are in the process of being taken over could command a couple eager generals to organize Black Merc Ordinator Raids on XCOM bases citing ANY reason. They don't really need a valid reason: illegal trade would be as good an excuse as any. Or "Ooh! You are hurting our Holy Space Brothers! Die!"
Not really sure it's worth it just for yet another slog through bullet spewing human enemies we've fought a few dozen times already. As far as I understand the code, this would require making a whole new "Alien Race" of hostile government troops, giving them an early game Base Assault condition and then never using them again. Granted, you could reuse a lot of assets you already have, but I'd rather not beg for even more of the same early game gunfights (It's a base assault, get ready for 30 plus turns of peakaboo). Especially if it takes away time that could be spent adding more of the planned content or polishing what's already there.
Not to mention the immersion fuckery surrounding why you have an underground base prior to promotion III. Yeah, you're a side-office underfunded government agency sending out agents with Berettas and cheap suits. By the way, here's the keys to Cheyenne Mountain. First base assault shouldn't happen until it starts making sense for you to have secret underground bases with actual hangars and elevators to the surface rather than second-hand offices and rental cars.
Base assaults were always the gut check "shit just got real" moment in XCOM. Where you had to face serious campaign ending consequences if you couldn't defend what you'd rightfully stolen. Throwing them at you early game just cheapens them IMHO.
IN OTHER NEWS.
I actually just wanted to ask about the math for stun damage. Just researched the KS-23 shotgun to find its stun ammo does LESS base damage than a standard shotgun's (22 vs 25). The only other way it differs statwise was 15% more HP damage. Does HP Damage Multiplier mean a percentage of the base damage is dealt as HP instead of stun or is it dealt in *addition* to the state base damage. So a 22 damage baton round is doing 11 stun and 11 hp damage (RNG dependant, obviously) or is it 22 stun and 11 hp.