If I could rework the Sidar (the only civ I ever play) any way I wished, here is what I would do
-Make it so they couldn't build cottages
I rarely build cottages myself, because I try to play the lore of an unseen empire.
-make forests, jungle, ancient forest and haunted lands spread faster within cultural borders, (i just feel they are spookier/ghostlier), have ruins, ancient towers and forts/castles/citadels give a food bonus (might make fort spam a problem). Again, they have that more spooky feel to me.
-Maybe make the razing of an enemy city provide some mane like units to add to population of Sidar cities, although I am not sure how lore accurate it would be to make the Sidar a city razing civ.
-I would suggest for the hidden city, leave the radius to 2, just forbid foreign trade for the city, and give maybe a 50% trade penalty. Internal trade isn't that great as it is. I see the hidden city more like a mirage city, hard to target, perhaps even part ethereal like ghosts. All units inside have blur, ranged units attack at -25% str., regular units at -10%, BUT magical weapons attack at 50% (enchanted blade is +30% str, flaming arrows, poison are +1.5 str, fireballs are double str, etc... basically any magic attack is +50% str.
-I like the idea of mist limiting visibility. I remember being so bummed when I found out units simply ran over my ghosts hidden in forests, instead of being "ambushed" by them. I am not sure the AI could handle moving forward in a tile with an unknown defender or defenders, while I am at it, I wish the blind spell did the same thing, like when units are held, their sight range is 0.
-it might also be interesting to make the Sidar not able to build workers or any improvements at all, except roads and those on resources.
probably a coding nightmare.... but if I could code, that's how I would do it.