How about simultaneous lauching? (haven't seen it mentioned before)
It is both realistic and a deterrent to the use of such weapons.
Regarding fun/realism/gameplay
Well nukes are always fun to use, exept when you get hit by them.
About realism, hiroshima and nagasaki were able to stand up, but that wouldn't happen with modern weapons I guess. However, creating different types of nukes seems too much detailed and out of proportion with the rest of the game. Also, in the name of realism, nukes should be fairly easy to build, and the result of a nuclear war is pretty much the end of civ.
So regarding gameplay, considering that we still don't know how the new military module will work, we haven't tested it yet, we haven't exploited its flaws, we don't know how the ai behaves, it is very hard to say how it should be. What we do know is that we'll have less units, either by higher production costs, resource requirements, maintenance, etc so we can expect nukes to follow this trend.
That said, nukes shouldn't be able to prevent a victory. Like, there you are after millenia of civ development, and on the eve of lauching your spaceship, ghandi levels your capital... same with a cultural victory, why should I strive for it, if all my major cities can be obliterated at the end of the game? (assuming the ai can perceive imminent defeat and is willing to do anything to prevent it)
I think nukes should have strict military purposes, reducing the city pop to 1 thus crippling their production but not razing it, reducing the city defenses to a minimum but not zero, and with a very conservative approach regarding the destruction of buildings so that it doesn't get overpowered. If whenever someone wants to lauch a nuke, every civ would then have the option to lauch their own (taking the side of the agressor or the victim), that could lead to an interesting MAD cold war scenario (again, assuming the ai perceives the threat coming from a nuclear war, and would always think twice before starting it).
The crippling effect on a nation's economy, and potential side effects, such as war weariness and/or unhappiness, should be devastating enough (for both sides), so that lauching a nuke is a last measure resort, and not the very first thing you do when starting a war.
About their range and such, I think we should keep it simple and have only one type of nukes, the tactical ones. Those that make you think where to place them, and give nuclear subs a noble purpose. Also, nukes should affect only one tile, since units will be scarcer and you shouldn't be able to wipe out a nation's army with one or two nukes (I do think however they should kill whatever unit they hit, it can give you the edge on the battlefield you just needed).
This whole discussion is pointless however, since by this stage of the game they already thought and hard coded all the nuking business, and all that's left are balancing issues.