So instead of changing everything from the UI, play style, environments. Why not just Add new planets in the next Destiny and keep the Tower in the same place that way it will be easier to adjust and will have a fresh feeling! So the first game has Earth, Moon, Venus, Mars and the Reef, next one will have new destinations but will have access to the Tower just like the first game. Now for stats I think they should be transferable between the two games and same goes for items in the Tower, You leave Destiny1 and pop in Destiny2 and in the Tower is all your items. If you really wanna make some extreme bounties and story missions you should have to play some story missions on one game and then finish the other half of it on the other game. Now for digital users if you didn't change two much of the game, just make the second game a add-on to Destiny. That would be very convenient and probable take a lot of stress of developers. Anyways fight well guardians!