Since Bungie are planning to make Destiny a very long term experience, they will almost certainly add new content over time, whether it be additional story missions or new planets to go to. Plus, as was said before, there is a lot of replay value to the missions (I found myself playing a few repeatedly during the beta). Though when the game finally launches, I wouldn't mind seeing some surprise missions or events that we have had no previous knowledge of. I believe there was a recent interview of some Bungie employees and they mentioned "Night Strikes" or something like that. Overall though, I'm hyped for the full game :)