My bottom line on the spy system:
If SD determines that sliders is the best that they have time and resources for, I would want it in the game FOR THE SAKE of those that find it important/ I would likely never use it, but there are certainly enough people that it matters to that it should be in.
I would really like to see one of the races be consummate spy masters, I think it was MoO or MoO2, they had a race that was abysmal at research, but it didn';t matter because they just let others do the R&D and they stole it. There were a lot of downsides, one thing was they were pretty universally distrusted if not downright hated. But their espionage was so powerful that it made them serious contenders in the game. The Darloks. Had to Google that.
Anyway, I don't even remember that many details, and honestly it may have just been slider driven espionage as well. I guess what made it good was it was not an afterthought, it really seemed to be well designed and thought out, and if you played them it sucked you in and was fun. I am sorry, but paying 30 credites per turn per empire to get notified by RNG of something happening or the occasional notification that you stole a tech, and also needing tp pay a cost per turn for counter espionage - not trying to be a loud person, but to me that is the opposite of fun.
Another thing that would make spies more valuable, and this has been brought up before, is other kingdoms/empires need to be a bit more secretive about their information. Why send a spy in to find out what techs they know, if you can just open a diplomacy screen, start a trade, and see a list of all their techs?
Good luck to you spy folk, I really have hopes that we can have a solid spy system that can deliver on the fun side of spying. I really think a great way of doing that would be to have a Canon race that specialized in be sneaky bastages. Being able to design your own race with those abilities is not enough imo, it just seems to "afterthought-ish".