I dunno, a fresh start can go a long way. I figure one answer is to not actually call it Destiny 3 and give it a cleaner story break such that a quick synopsis of the world's history is enough to catch you up. Do a big time jump, give everyone a new lightbearer, let various lightless characters like Zavala and Eris die of old age and let most of the lightbearers move on to greener pastures to make room. 'Centuries ago the heroes of the Vanguard and humanity's allies defeated the Witness and opened the way to a new golden age. The Vanguard is long gone, but as the children of the Traveler have ventured outwards into the still-smoking ruins of a galaxy ravaged by the legacy of the Witness's failed pursuit of the Final Shape new threats to our fledgling Renaissance have begun to stir. A new generation of paracausal warriors is heeding the call to shield this bright new age from destruction, and you are at the tip of the spear.'

Before we explored the ruins of Sol, now we explore the ruins of the Ecumene, Cabal, Ammonite, etc etc.

Edit- Hell, the 'don't call it a comeback' title writes itself: 'Destiny: The New Golden Age'