Never thought about this... even when it's like Athens' historical connection to its port (too bad in the game one can build docks on neutral territory, I'd reserve that just for some civs):   Regarding the baking of champions, it does seem to me that's a bit OP, but I don't play MP. Related: why is it that a building cannot be set as its own rally point? Maybe because this strategy in general would become too OP? And yes, Hippocrates is the best, the AI can't deal with the nigh immortal army.

An intermediate position, rather than making it invisible would be to deactivate the unit coloring, making it harder to spot when in a forest. But idk if its a good idea, sounds more like scouting and less than spying

An alternative could be having a spy unit that only the owner can see (or the enemy would see as its own), along with enemy Lookouts and maybe Towers, also becoming visible after performing some action. These actions could be bribing, revealing information usually hidden to the player (enemy garrisons I think? or whatever useful), assassination, Hero detection, infiltration (climbing walls and palisades), and opening gates, among whatever others can think of. These actions could be made easier or unlocked with experience (instead of having extra techs that will be hard to historically justify). I'd rather have added micro and complexity from more interesting things as these than from repeatedly clicking easily automatable stuff.

What do you think about an enlarged Spying tech branch in the Tech tree that includes a Bribing soldiers tech after a basic Bribing merchants tech?

Super this is great. I will close my PR.