May be rather complex, but how about fuzzier thermal detection? For instance, a task group appearing as one big thermal signature rather than many distinct ones at the far range of a sensor and perhaps even blurring together signatures of nearby ships at the furthest range? Nearby signatures would still be rather distinctive, but i am having fun imagining this if stars also maintained a signature as well.
I think this would be perfect, for lower tech. I think, yes the passives should be changed to be a little less clear, that it would be easy to distinguish between multiple small contacts and a big one at a medium/high tech level. It is an interesting thought for the mixing the signatures, and that could be put in as a TG order to fly in
close formation to mix signatures (has some risk/adds additional hit chance on those ships (enemies firing at close formation ships/TG get a hit chance bonus)). And I think that another tech should be added with this,
Sensor Clearness (name debatable) that does exactly what is says, a multiplier to how clear passive contact(s) are (affected by close formation).
A side note for another feature that would go well with this is possible contacts. Based on their speed and thermal (and EM if aplicable) it would display the most probable contact (based on percentage somehow). This would be with the aforementioned sensor clearness (to make the %) so it wouldn't be automatically based on number of similar designs (ie a 4000 km/s 4800 thermal beam ship, a 4000 km/s 4800 thermal missile ship, ect.) (also affected by close formation). Also changing he ship speed in the orders would change the percent as it gives a different speed and thermal than what the ship is capable of (but still a possibility as that ship is capable of that s/t because of speed scaling).