I can only speculate, and in no way this is a right or wrong answer. Even though destiny is not an MMO, they do have the idea that they want a "persistent world." I think you won't get Destiny 2, 3 and other sequels, but instead get expansions like in World of Warcraft. Again this is pure guess work, but it seems that bungie plans for Strikes and certain challenges to be released over time after the game's release. The idea is that there is always things to do in the world. And there might be major releases that bring in new planets, new stories or even new classes. Now what we need to know is what we are going to pay for this. Is there going to be a season pass? A subscription model? I think boarderlands is the closest game, mechanically to destiny so you might want to look and get some ideas of what the game's progression might me. Like you I'm looking forward to the day when Bungie lets the cat out of the bag and we get some more info on the game.