I'd like the video to be unscripted, you can write down the things you are going to talk in the video, but I don't think it will flow naturally if you decide to script it.
Either do it live in game while you're playing. Or do voice over after collecting a number of interesting clips together. I think that would be a lot more interesting, spontaneous and less dull, especially if you can figure out interesting topics to talk about while walking around a town or in the wild.
I duno, but i've watched a lot of minecraft video where the guy hosting talks about a lot of funny things and manage to make an interesting video even though all he might do is just run around killing things or building a house.