What about allowing the use of cruise missiles after you Denounce someone, but without having to actually go to war?
I think you're getting too wrapped up in game dynamics and overlooking the realism/role-play aspect. When nations are launching missiles at each other, they are effectively at war.
But I think I'm getting where you're going with this- I'd like to see more options for peace-time acts of attrition against other civs. First, through embargoes, and second, from a more enhanced espionage system.
For the embargoes, you could do things like pillage trade routes (although, it's kind of self-defeating since most players use internal trade routes and if you're making peaceful attrition against a player, you probably don't have OB, but I digress...) You could set up troops along the border for an amenity penalty against the other civ based on the total combat strength of the units at the border, For a shared border with units on both sides, there's a significant reduction in the penalty for the stronger side. Also while under embargo, the total combined military strength of both nations is weighed against each other, and the nation with the lower total combat strength suffers a penalty of 1GPT per X difference in combat strength (where X=10? 5? 1?)
As for the espionage, I'd like to see spies doing what they're doing now but to a much larger scale, have more of them available, maybe different classes that allow better defensive spies and specialist offensive spies, and either way a LOT more of them available IF you make the investment in them (maybe requiring a district, I dunno- something that would redirect resources away from conventional development towards espionage advancement.) Spies would have different abilities based on the relationship of the two civs:
alliance: not allowed to spy on an ally
neutral: can take actions which are a minor annoyance against the other nation (production sabotage, research/culture progression hindrance, etc.)
denounced: the above actions are more potent (bringing production down to 0 rather than an X-hammer fallback) and spies can now perform actions which benefit the player as well as annoying the target (gain eurekas for techs they have/you don't, siphon funds, etc.)
embargoed or at war: under these settings spies can do powerful things against other civilizations, like reduce the population of a city, use propaganda for a massive shift in loyalty, and my favorite- strategic sabotage on select district (if successful, each attack pillages sequential district buildings until there are none and the district itself is pillaged.)
It's been a long time since a civ game has had a descent attrition element to the game: the last was the privateers of either civ3 or civ4 where you could use these units under a neutral flag to attack other player's ships. Since then, we've had various attempts at espionage most of which had spies be ridiculously expensive or have actions that cost obscene amounts of gold. I'd like to see elements like this in the game for several reasons. For one, if you're playing aggressively (which is usually more effective) you're going to get lots of denouncements from other civs (unless you're the absolute master of the diplomacy modifier game.) which for you just means you're only able to get garbage trade deals from that person. It would be nice if denouncing came at a price; it enabled the person being denounced to start working against the cold-war opposition. I always thought that real-world public denouncements were rather rare and much consideration went into them, there must be a
cost that is being considered, whereas in this game they're pretty willy-nilly about it. Second, it adds a second layer to how civs perceive each other as threats- Currently, they just look at who has the most guns and the most advanced guns and conclude, "don't mess with that guy" or "we need to get people in a joint war against that runaway." Adding this espionage element brings the decisions to a new level, like "My military is WAY stronger than his, but if I attack, his WAY stronger espionage is going to completely rip up all my uinfrastructure." I think it would add new levels of game-play.