If Destiny is an MMO, it won't have a sequel. Sequel to MMO systems are killers because they split the base and make both games smaller. If Bungie knows anything about the market, they will never think about doing a Destiny 2.
They'll make a Destiny 2. If they're talking about ten years of Destiny then there'll be a Destiny 2. Probably because Activision will make or them or maybe because they don't know how to make an mmo.
I try to get to the end of each section of Tartarus in as few days as possible. That part where you can't go any further because the floors are blocked off. If you can make it to the end then you could probably stop going to Tartarus without worrying about your level.