TL;DR: Build a national project to launch 1 nuke per enemy city, regardless of current MAPs.
12 MAPs to launch a single nuke that might get intercepted. It just doesn't seem worth it with today's mechanics.
Proposal: Create a national project called the Nuclear Triad which allows you to launch a wave of nuclear missiles, one for each opponent city.
Nukes are a joke, no one is going to store up the MAPs to use just one nuke when the enemy is pounding you with ground, sea, and air strikes. This project/mechanic would not only make nukes viable again as both an offensive strategy, but also as a deterrent. This project is also already balanced in that the initial cost for nukes, as well as upkeep costs are relatively high.
The function for the number of nukes to retain is f(x) = (opponent_max_cities + 3_spy_ops). If the opponent has the Vital Defense System then f(x) = (opponent_max_cities * 1.2 + 3_spy_ops)
For a simulated opponent with 20 cities, the VDS, and spais, the number of nukes needed to ensure a complete second strike is 27 nukes. The initial cost would be 20k alum, 12.5k gas, 6k uranium, and 47 million dollars. Upkeep costs would be 945,000 per day in peacetime. As the game currently stands, I believe this is balanced *enough*, although thoughts and suggestions are welcome.
For a brief description of the Nuclear Triad project, click here.