1) Disable Gambit-related seasonal challenges, forcing us to waste time in tedious activities (maybe add them after the Gambit overhaul).
2) Make PvP grind-worthy.
3) Add missions like "The Whisper" (this mission's soundtrack, being dark themed and finding yourself into some deep, hidden and dangerous place... was genius) and "Zero Hour" (puzzle jumping and traps are actually FUN).
3) Add exotic sparrows/ghosts/ships which can be achieved by completing hard triumphs, like Harbinger's Echo, Sagira's shell and A Thousand Wings.
(it's infinitely more rewarding to acquire something/anything by doing hard content/triumphs than buy it from the shop).
Same thing for emblems and shaders.
4) Uhm, maybe fix shaders? ...most shaders' colour palette does not reflect it when put on an armor piece. It's difficult to have 3-4 different sets looking the same while using a single shader.
5) Seasonal activities (wellspring, ketchcrash, heist battlegrounds) aren't that fun and I don't think people would do them if not for the seasonal challenge perk unlocks and red borders for crafting.
They're usually linear activities. You start from point A and need to kill mobs to point B.
Repeating content should have some stimulating element at least.
6) Implement a system to prevent trivializing content.
You shouldn't be able to clear entire rooms with little to no effort while in hard content you die in two hits.
Doing 3 weekly strikes for the pinnacle is like a test of patience to survive the bore, while doing the 100k legend nightfall is more of an objective-oriented activity in which you work actively to achieve.
7) Lag in PvP... just fix it soon and for good, without collecting data for one year before trying an experimental method, attempting to hopefully reduce it slightly.