Nice to see that Galactic Civilizations 3 sees introduction of new counter-opposing combat stats for ships: accuracy and evasion. However, ever since GC2 times I've been wanting to suggest adding a steath ability that would be playing off against sensor stats of other ships and would add a new cool edge to ship handling and military displacement outside of direct combat.
How could it exactly work? Very simple (in case we're scaling it with sensor stats): every point in stealth decreases the range at which non-allied sensors will show our ship. Let's say our ship has +2 stealth and enemy ship has +5 sensor range: thus enemy ship will be able to see us only at the distance of 3 (as if we had 0 stealth and enemy had +3 sensor range).
I hope you'll like this idea and I suppose it isn't something really hard to implement in the game. This ability will of course require adding technologies that will unlock corresponding ship modules and even provide a global stealth bonus to all your ships, plus a global bonus or penalty that can be chosen during civ creation.
Just think of all those new opportunities to sneak up on your enemies to strike into the weak spots or just evade combat when you don't need it, and the elevated importance of sensors in general and AWACS-like monitor ships with lots of sensor modules that will be necessary to counter enemies with high stealth.