I'd like to see a Stealth defense module for starships, starports and starbases. And planets. The counter for this would be research along the Sensor's tree.
Stealth would obviously keep the other player from seeing the object in certain situations as well as affecting the targeting of ALL weapon types. Stealth could also be a mini-research tree like sensors with multiple levels of effectiveness.
E.g., my ship with a high-level of stealth would be completely invisible to a player with no sensor research and if I attack them they're severely degraded in shooting back. At the other extreme, I have low-level stealth and they have high sensor capability, so they can see me, maybe only at closer ranges, and their targeting is closer to normal.
Perhaps some races and non-player groups (e.g. pirates) might have a native stealth capability or reduced cost to research.
I assume this is kind of abstracted into the various weapon and defense types, but breaking it out would extend it into the intelligence and snooping around arena.