They need a bigger loot pool methinks (in addition to stuff mentioned above).
This is exactly what they need. In addition to that, the loot at Legendary level and above also needs to all look unique, distinct, and be desirable from an aesthetic perspective alone.
Why do all of the legendaries look like mostly bland, Earth factory-made guns? This is supposed to be a sci-fi epic set in a universe with technology and magic beyond anything we know in our reality! The guns should, for the most part, look nothing like anything you'd see on Earth in 2017. The vast majority of the armor looks like nothing special as well, and again—there are far, far too few sets.
Mose loot just doesn't look unique/cool/desirable enough, and there's just not nearly enough of it. There needs to be highly desirable, rare loot for every activity to make them worth doing, and lots of it too:
1. Full armor and weapon sets that look unique and desirable for each class for every single Strike, Public Event, and Lost Sector; no reskins
2. Full armor and weapon sets that look unique and desirable for each class for the Nightfall and Prestige Nightfall, as well as for the Crucible; no reskins
3. Did I mention no reskins???
Bungie's got to be the most new content/new asset creation-averse AAA developer I know of. It's a combination of laziness and what is, I'm sure, penny-pinching enforced from on high; Activision suits are no doubt partially to blame.
Bungie, stop with the low-effort, lazy reskins and just do that thing called putting in the work to make actual new and diverse assets. The sheer shallowness and lack of creativity, visual uniqueness, and aesthetic appeal of D2's loot pool is unacceptable.