It's not surprising at all that the follow up game is in development. I'd image that it has been for quite some time. We know that Destiny has a pretty large development team, so I assume that they're split up to have people working on all aspects of teh project. That means that work on Destiny 2 won't be taking people away from work on the current game or it's expansions. That's not how work on projects like this is usually divided. So this shouldn't be a surprise for anyone and it shouldn't have any effect on stuff like raid fixes, PVP balancing, DLC content, or any other aspect of the current game. There's no reason to be alarmed or concerned.