Bring back Destiny 1 Planets/Destinations as free DLC.
This would be a great way to keep the community invested inbetween paid expansions, and also offer something more to players who don't buy the expansions.
For example; the Cosmodrome and the Plaguelands. They could be combined into one destination, and upgraded with brand-new Adventures, Lost Sectors, and maybe even a Strike or two (like the Mahlok strike in the April Update for D1), and also bring back some iconic Destiny 1 Exotics like the Khvostov, Queenbreaker's Bow, Dreg's Promise, and Gjallarhorn, all of which would be obtainable only through Exotic quests in the Cosmodrome.
And they could keep doing it, over the course of Destiny 2's lifespan between expansions, adding in more D1 destinations as free content and making them better... the Moon, Venus, Mars, and the Dreadnaught. Each one bringing more Adventures, Lost Sectors, Strikes, and Exotics, all free for all players. I think it would breath new life into Destiny 2.