Basically, it will probably be like what happened in TTK. We didn't really have to "start fresh", but with the expansion there were green drops more powerful than the current legendaries we had equipped. So I imagine it will be something like that. Our guardians won't be reset, but our stuff just won't be powerful enough to use in Destiny 2. I'm sure they'll drag it along like TTK too by reintroducing exotics in increments and expansions. There will likely be some changes to subclasses as well. I can see us being sapped of our light or something as an excuse to reset that kind of progress, but we find and activate new more powerful subclasses as a result or something. Anyways it's all speculation, i mean, at one point I don't want to lose any progress and I'd like to use everything I've earned over the last couple of years... But at the same time, in a new game, a sequel, it's kind of hard to justify not starting from a fresh point.