I was thinking it would be interesting if you could promote an intelligence officer to a spy (or maybe just have the spy as a standalone).
A sub with a spy would have to be twice as close to an outpost before appearing on the map. This would add a good level of tension to the game that wouldn't be too ridiculous but would always leave an area of doubt in the back of everyone's mind. It's also a good way for those who are behind to make a great move and come back from the grave.
An interesting attribute of this would be that having an intelligence officer is good for ratting out enemy spies because of the increased sonar range. Essentially spies and intelligence officers would somewhat counter each other.

Perhaps there would need to be a cap on the number of drillers (maybe 50 - 70) that can be on a sub with the spy on it. That way players can at least prepare for it in some way without having a massive army suddenly be right next to their queen.

This "hidden sub" idea has been tossed around a bunch, and I generally think it's a bad idea.
However, I will say that your idea is the most reasonable I've seen. I support it.

I can't say I'm too keen on a hidden sub unless it cannot be used to attack. Maybe this hidden sub can offer a very small sonar around it and be used for recon? It can be sent towards an enemy outpost and when it arrives there it automatically turns around and comes back at 2x speed with its small sonar disabled. This fits in well with the Intelligence Officer and makes it a fitting promotion.

Champinoman wrote:I can't say I'm too keen on a hidden sub unless it cannot be used to attack. Maybe this hidden sub can offer a very small sonar around it and be used for recon? It can be sent towards an enemy outpost and when it arrives there it automatically turns around and comes back at 2x speed with its small sonar disabled. This fits in well with the Intelligence Officer and makes it a fitting promotion.

That would work well also. A hidden sub is indeed kind of a sketchy topic because it can be used dangerously if it isn't balanced carefully