I've been thinking about this for a long time and Id like to see a significant expansion to the nuclear option, reflecting the way that the weapons technology has snowballed over the last 70 years.
1. Nuclear missiles would become available once Nuclear Fission is researched, instead of requiring Advanced Ballistics. However, they would be weaker (initially) than they are currently. Theyd only require 1 uranium and their range and destructive capability would be the same as (or even less than, maybe) the atomic bomber.
2. Upon researching Advanced Ballistics, atomic bombers should become upgradable with a Strategic Air Command promotion, purchasable with gold. This promotion will allow half (rounding up) of the promoted bombers in any city to be considered to be in the air at all times. Hence, some bombers will survive a nuclear strike.
3. Also available at Advanced Ballistics would be a promotion, purchasable with a second uranium and gold, to bring the nuclear missiles up to ICBM standard. This would raise their damage capability to the current level and double their range, making them much more scary.
4. Upon researching Nuclear Fusion, ICBMs would become upgradable with another range and damage promotion, purchased via gold. This would reflect lighter payloads and higher yields.
5. The Satellites technology would include Mutually Assured Destruction. Once researched, any first strike nuclear launch against the civilization would allow a full retaliatory launch against the aggressor, followed by any further launches from the attacker. All damage from the exchange would be resolved simultaneously, once the launches had been committed. This retaliation capability would be limited to nuclear missiles, ICBMs and SAC promoted atomic bombers only but would not be limited to weapons from the attacked city.
6. The use of nuclear weapons, even in retaliation, would risk huge diplomatic and trade penalties. A special session of the World Congress would be convened with a vote to embargo trade with both combatants. The default position of all AI Civilizations would be to support such sanctions against the aggressor but this would naturally be modified by social policies, ideologies, friendships, etc. The default position of all AI Civilizations would be neutral towards such sanctions against the defender but this would again be modified by social policies, ideologies, friendships, etc. It would also be loaded towards sanctions if the retaliation had been disproportionate.
Well, that post ended up a lot longer than Id expected. But I reckon something along these lines would allow the nuclear capability to escalate alarmingly through the end game and introduce a real sense of brinkmanship, with the possibility that the two combatants could bomb each other into irrelevance.
Feel free to rip it to shreds,