Instead of just throwing more and more raiders at a base, have different types of events show up. You guys are constantly nerfing defenses so that raids can't be cheesed. So you're worried about stopping players from doing what works. What you're not thinking about is how raids can be changed so that they're more dynamic and players need to invest in defense strategies that are different and dynamic to respond to different kinds of threats.
Instead of asking "What do I nerf next?" - you need to be asking what can be different about raids, enemy units, equipment, etc.
Maybe there's an alert that a suicide bomber has been spotted, he's going to touch a wall and then explode. Maybe flag that enemy so it's easy to see.
Mid-raid events: 20 tribals show up and attack from a different direction.
Two saboteurs have dropped into a high value room and are going to begin destroying high value objects or maybe they'll just steal something and then mine out. This gives players challenging decisions to work with and new objectives.
A faction leader has just dropped in and is joining the raid. If you can capture them,
A special centipede with super armor just showed up. It's primary function is to take lots of damage and cause the player to focus on that while managing the rest of the raid. It's virtually immune to all forms of traps and security weapons. The only way to take it out is melee.
The nature of the raid just changed. It looks like they were about to do a regular base assault, however now they're building mortars from an airdrop of materials that just arrived.
Mounted troops: Cavalry are particularly deadly in melee, you don't want them getting close. They're awfully fast, so they may try to flank your lines or they may act like mounted archers did in real human history. They'd attack, then run away, then attack, then run away. But they run so fast you can't catch them on foot.
Undercover enemies. Some enemies arrived in stealth and they're already walking past your walls and some of your defenses.
Spies. Some enemies have arrived which won't be targeted by your defenses or your colonists unless you can have a dog or trained animal sniff them first. (Command and Conquer Red Alert had something like this.) - You can borrow from other RTS games to get GREAT ideas.
Oh my God, those aren't traders, it's a raid! - Traders enter the map and slowly approach, just like normal. But after arriving at their destination, they draw guns. Their goal is to either steal your goods or do some kidnapping.
Parley. Raiders arrive. Right before the battle is about to begin, one of them lights up like a quest giver. A good negotiator may be able to intimidate them into leaving, or bribe them to leave at a low price. Maybe, they can even improve faction relations by establishing a trade. Shotgun diplomacy to meet their need for 2,000 corn. Will you agree to let them have it or will you agree to a trade request that must be filled within five days? - That sort of thing.
Bugs are tunneling up over there! - Bugs begin spawning at a location on the map. Not all at once, but in a flow. Let some suspense build up. Maybe it's some kind of underground bug super-nest or tunnel that's going to take a lot of firepower to destroy. Until it's destroyed, bugs are going to come out of it.
Solar flare! - The enemies have either released a super sized EMP or they've created a solar flare event in the middle of their raid. Your electronic defenses are going to be down for a while.
A faction leader has arrived on the map mid-battle. All enemy combatants within 16 tiles of them are going to fight at +5 to all of their combat skills, their pain threshold is increased, their movement speed and mood is boosted. - You might want to get a sniper to take down that leader, or use a shock lance on them.
A drop pod has arrived containing a super weapon. Enemy raiders are chasing it down. Can you get to it first or are they going to potentially nuke your base with an ancient weapon?
Dead man's triggers: The enemies have sophisticated bombs planted inside of them. They're all going to explode when they die. Try to shoot them before they reach your walls. Move them away from anything valuable. Maybe engage them outside. So many strategies and problems.
There's a few zombies going on. A few enemies will resurrect several times unless burned.
All kinds of things can stop a battle from being static where the player just stays in a certain area and waits to shoot oncoming raiders. The goal is to have a mix of dangers and opportunities. Also, that strategy needs to become obsolete for certain enemies. Ideally, any of these should have the potential to do enough damage that a battle is changed or that base design and player behavior needs to adapt for it.
Most RTS do this through dynamic units that are balanced with wildly different stats like maybe some are super fast, some are super numerous, some fly, some have excellent melee but weak shooting, etc.
Rimworld doesn't have many distinct enemy types.
There's humans, bugs, and mechanoids.
Maybe it's time to introduce some kind of archotech or even a type of civilization like the borgs in Startrek.
Infected. Raiders are going to have bugs burst out of their bodies eight hours after death. There's no warning about this ahead of time, it just happens unexpectedly. This doesn't address the raid complexity problem by a lot, but it's a fun event that players suddenly need to think about. Maybe they'll be more vigilant about corpse management.
Something like that makes players worried, and you want them to worry. You want them to feel unprepared, unbalanced, anxious, anticipatory. The idea of a trade caravan turning into a raid is a perfect example of that. Suddenly, the rim feels dangerous. Every time a caravan comes in, the player is acting cautious instead of feeling completely relaxed.
Message: A trade caravan is being chased by raiders. Offer to help? Raiders will arrive shortly after. - Create situations where raids have to be met outside of the base and away from defenses. Now players are going to think about building defenses all over the map and making a variety of strong points.
Message: A trade caravan is under attack outside of your base. They need your help. Will you send colonists into a battle?
Message: Thank god you've developed some drop pod launchers. An ally needs help defending their base. Please drop into the middle of it. - Now a player can experience defending a base using defenses that aren't their own. They can't rely on killboxes to win this one.
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