On the concept of silos what advantage does it provide. Remember the reason for silos was not to hide you nukes, being fixed installations they would eventually be found missile subs served that purpose. The advantage of a silo is that it is hardened to attack, allowing you sufficient time to prep and fuel your ICBM. Now as I understand it, ICBM's are not destroyed in a nuking of a city.
Hence a silo improvement would not be adding anything new to the game. Considering that the game already abstracts things. The ICBM's in cities are already in silos some place in the surrounding tiles of that city.
Now if you wanted to add a new nuclear element, what you want are trailer launched strategic nukes. You would be able to move these around, keeping them hidden from your enemy. Move them close to the line and launch a surprise strategic attack.
Don't know if it is the case with the current strategic nukes, but at least these trailer launched nukes would represent tomohawk or scud missiles and hence would not be candidates for the SDI to shoot down. (giving them an advantage over ICBM's, which are otherwise pretty much hands down better, well except for the build cost).
