I don't know about anti-satellite missiles, hadn't thought about it yet. Space based weapons is a tech in our tree, right now it just leads to SDI. We could add an anti-satellite missile that blows out another nation's advanced satellite project, but this sounds like a lot of code (and the standard AI use problems that follow) for one unit. If someone wants to build this and can get the AI to use it, it would certainly add to the game though. Maybe something to focus on for version 1.5.
As far as missile defense, we have two types. Strategic missile defense, which will require a building in each city. It would be nice to also have a mobile
Patriot missile unit that increases chances of interception and can shoot down missiles in areas that don't have the building. This is meant to represent existing surface to air missile defense systems. I haven't found the specific building(s) I am going to use for this, so please feel free to search around for an existing one and post your suggestion(s) in the regular buildings thread.
Then later, you'll be able to research and build the standard
SDI, which is space based missile defense and will obviously be more effective. This will just require one national project (same as it does in vanilla). No one will start with this.
One of our nuke mods already modifies SDI so it has three levels, so we can do this for either or both of our missile defense systems. We can decide the exact interception rates once we're testing. For it to be accurate, we have to make the chances of interception greatly reduced by larger strikes. The more missiles fired in one turn, the lower the chance of interception. The first missile defense system will have almost no chance of shooting down 2 or 3 of the most advanced missiles, for example.
And you guys are right, these aren't going to be unique to any civ.