I have alot of friends that are going to be playing Destiny, much more than 3. I am probably not alone with this. So I suggest that you increase the fireteam size, atleast to 4 in story missions, and for explore make it like 10 per fireteam. Or say i have a group of 3 friends in a fireteam doing mission 2 in old russia. Make it so that I can join their world with my own fireteam of 3. Somehow you need to make a way to play with more than 3 friends on a mission.