TL;DRAlien battle tactics need a serious overall. After a very early point in any game, it's easy to forget the aliens are even there because they don't scale and a few walls and basic turrets can end them. You don't even need rockets or piercing ammo. Here's an example. This was so easy to set up I never upgraded from basic turrets:
- Direct attacks on alien bases should really cause the aliens to start reproducing and spawning new areas - and maybe causing them to try to gain ground near or in your base, much like in the tutorial. That tutorial was how I feel the game should play.
- Gigantic walkers that can use a charge attack to breach walls would be a game-changer. These walkers, when killed, could maybe spawn a wave or two of units until you also destroy the corpse or they run out of material. The main goal of the walkers would be to breach your walls or find an area to burrow and become a new alien base.
- Hostile flying units would be good. They would ignore turrets.
- Burrowing snakes that can go under walls?
- The alien bases go from tough targets early on, to something you casually wipe off the map at extreme range. I think there need to be more consequences for artillery. Piercing ammo seems to be more effective than a tank's main gun with an explosive shell - which has low range, making it easier to rush everything with your machine gun blazing. and there is no pressing reason to upgrade to uranium rounds. There is, unfortunately, not much reason to use the tank flamethrowers at all - except, maybe, as a hold out weapon. With repair drones, the tank is unstoppable - the only way to kill it is for the player to run out of repair packs.
- Abandoned human bases with hostile defenses would be amazing. There is already a mod that does this, and I think it should be integrated. Could have automatic builders. Would mainly be fairly basic failed mining/military bases/outposts with most of their best stuff already destroyed - but maybe with a few rare finds.
- Ancient alien bases automatically mining minerals and making defenders. Would allow the introduction of unique artifacts and structures the player can't replicate. Some of these could be required for high tier research (higher than is currently in the game, even), however. Would also open up intriguing puzzles - maybe randomly-generated ruined 'mazes' with traps, and so on?
- Water-based and amphibious enemies would mean a greater level of threat, and would prompt the player to build flying or water-based craft. (I'd suggest gyrocopters & hover craft). This would also open up water-based resources.
- New triggers for increased aggression, such as reaching certain technological points or blowing up a certain number of their bases. Shooting them with artillery could case massive expansion, maybe.