Deeper class system with more abilities, more than "Melee, Grenade, Ultimate" that all classes had in some variations, and opportunities for synergy between players. Also, more weapon variations with weird abilities or more alien design and feel, something that shoots anything else than bullets. These two things are whats left for me on my Destiny 2 wishlist.
If they go with bigger worlds to explore, interesting lore and story, PC version and preciously mentioned stuff, it basically then checks all my boxes for a sell. Looking forward to seeing how it turns out.