As an extension of Walt's plans and the OP's suggestions, here are some additional part and gameplay ideas for boarding (and counterboarding):
Parts
Weapons Locker
- 1x1
- One door only
- Allows up to four or six crew members to equip themselves with small arms on-demand, just like fire extinguishers
- Suitable for smaller vessels
Armory
- 2x2
- One door only (having fewer doors means less avenues of attack for enemy boarding parties)
- Permanently assigns up to 20% of crew as always-armed security personnel, up to a maximum of 12 per armory
- Suitable for larger vessels
Ship Interior Defense System
- 1x1
- No doors or partitioning walls
- Can only be placed in corridors
- Crew can pass through (assume it is "ceiling mounted")
- Requires one unit of energy as ammo (similar to the Point Defense System)
- Automatically attacks enemy boarders within 10~15 squares
- 360 degree coverage, minus obstruction by ship internal compartments
- Can only attack one target at a time
- Can be disabled/destroyed if overwhelmed by multiple enemy combatants
Boarding Pod
- 1x2
- One door only (at the back of the pod)
- Room for four security personnel
- Infinite no-build zone extending from front of pod
- Automatically launched if target vessel is below X% health / structural integrity, leaving behind an empty berth/bay
- Autonomous flight and return
- Prioritizes targeting of any (hypothetical) docking ports on enemy vessel
- If the enemy vessel has no docking ports, the pod attaches itself to any corridor segment(s) and requires a short amount of time to break into the enemy vessel
- If the enemy vessel is fully clad in armor, the pod attaches itself to the armor segment(s) closest to the enemy vessel interior and requires a longer amount of time to break into the enemy vessel (by "converting" the armor into a corridor)
- Pods can be shot down
Gameplay
- Crew automatically arm themselves and engage the enemy if hostiles board the vessel (no need to toggle a specific alarm/alert condition)
- Small arms cannot be upgraded (Too OP? Too much micromanagement?)
- Each small arms fire shot follows a Gaussian distribution probability curve, where there is a low chance of dealing negligible damage, a high chance of dealing moderate damage, and a low chance of very high/instant kill damage
- The restrictions imposed by the armory will force players to think about provisioning small arms for their crew (e.g. is it more cost effective to have lots of permanent security personnel, or a small core security team supported by ordinary crew that arm themselves on demand?)
- Alternatively, weapons lockers may restrict the crew to defend themselves, whereas having an armory would unlock other offensive parts like boarding pods or teleporters
- In addition to boarding pods and teleporters, standard docking ports could also be used to forcibly dock with an enemy vessel that also has docking ports - an interesting player strategy could involve designing ships with docking ports recessed relative to hull segments to make it more difficult for enemy docking attempts.
Personally, I think that teleporters are potentially too OP, so there could be limitations (e.g. high energy requirements, effective range, quantity of crew transported) to help make other methods more viable