I really like the idea. However, submarines would then quickly be overpowered. There needs to be a disadvantage when attacking neutral units.
I think what would prevent subs from becoming overpowered is simply their relative weakness as offensive units. Subs have no problem taking out single units, but even moderately sized stacks of infantry quickly overtake them. To use them effectively in this manner you'd have to avoid large armies, making this new feature quite limited.
Does how long a spy has been active in an enemy's territory effect how effective they are at accomplishing their mission? For example, if you deployed 10 spies right before the day change, will they have a lower probability of successfully completing their mission?
Several weeks ago I finished game #7140580 having met all the requirements to unlock the "Economic Dominance" ribbon. All my territories were above 82% morale and I built over 600 upgrades just to be sure. I played the round specifically to unlock it. This is my third time meeting the requirements.
I waited a few weeks for the next stats update. While everything else updated, I still hadn't unlocked the "Economic Dominance." ribbon. Is this some kind of glitch, or is there some extra specification not listed in the Ribbon's description?
Giving subs the ability to attack neutral players would add an interesting new tactical/espionage aspect to the game, not to mention an additional layer of historical accuracy. In WWI, subs sank neutral cargo/civilian ships frequently; famously, the Lusitania.
It could be useful in situations where an opposing player is becoming a threat but you don't want to risk an open war with them yet. You can covertly hurt them a little-by-little by having your subs attack their sea-units without them knowing it's you. It adds a new level of risk to sending troops unprotected overseas, and makes players play 'detective' in trying to figure out who is sinking their ships.
Propositions:
