I have several Ideas:
Pitfall/Spikepit traps- you could even create a string of these around your base to act as a defensive trench instead.
trainable boomrats- basically mobile, tracking mines that you could unleash on a group of raiders as soon as they land. yes, your boomrats would die, and they should take a LONG time to train up, I'd say at least a full cycle to be able to use them this way. there could even be an animal husbandry trait that reduces training time by a certain amount. you could choose what you wanted to train in a screen similar to the research screen.
Fixed cannons- They could cost 500 materials each, and only have a 90 degree turning radius, but could do 75 damage per shot, have an affect similar to a grenade, and take 2 to 3 times as long to fire as the auto turret with around twice the range.
pet squirrels- They could be used as a happieness modifier. the animal trainer/animal husbandry person could go out and catch squirrels to be put in a cage built in each colonists room.
Taser- A weapon that does almost no damage, but can be used in melee to incapacitate enemies to be captured and recruited/sold/beaten/executed. Not dropped by raiders. Only acquired by buying from arms dealers.
Stungun- same as the Taser, except that it doesn't have to be in melee to be used. Not dropped by raiders. Only acquired by buying from arms dealers. Rarer than Tasers.
Electric Fence- self explanatory
Random ideas-
Raiders could demand tributes/protection payments of resources or weapons i.e.
"We hear you've got some good guns in there. Either you give us 3 of your Pulse Rifles or we put holes in you and take them anyway!"
Sorry for any doubles btw. also had another idea.
what if raiders didnt always decide to come in through the front door. if you obviously have awesome defences with your base inside a box canyon with only 1 narrow entrance which is filled with mines and covvered by 15 turrets, then maybe the raiders will try to tunnel into the back of your base.
this could make mining efforts much more dangerous and unpredictable. you would have to decided if you REALLY wanted to mine that big vein of metal and risk your defences, or do you want your rock wall to stay 15 squares thick.
I also think you should be able to fill in old tunnels with new rock in order to prevent pockets that aggravate my ocd. you could then fill in any tunnels created by the raiders and even wall off old mine shafts that have been mined dry.
also, I think you should be able to completely wall in something and not have a roof automatically form over it. I would like to be able to wall in my solar panels to protect them without blocking off my access to sun.
maybe a group of raiders could even drop directly into your base as an ambush. this could help prevent it from being a turret defense game. you would need to think about firing angles, placement of mines and even which directions building doors face and where your conduits are.