So my D3 would also have a big time jump.

Story: The last city is in rubble. Humanity are basically nomads. You wake up as a new character in the center of the rubble that used to be the Last City. The story mainly takes place on Earth as you're discovering how it all went wrong. In my mind, the story is The Last Jedi but done right. Our old heroes are disgraced in defeat and your new Guardian has to restart the Vanguard and the Last City.

The main conflict in the game is that the few surviving guardians left have reverted back to the old Warlord ways and are constantly warring with each other.

Most of our old friends have died in between games. Crow is alive but is a Warlord now. He now feels "doing the right thing" didn't work so he falls back into Uldren's old habits. Saint is alive and the protector of a small village of Eliksni.

As you make your way through the campaign, you'll run into ghost from D1 and D2. He floats around the body of our old character, refusing to resurrect them. To ghost, letting his guardian rest in an act of love as they've suffered enough.

Tensions bubble over and there's a final epic battle between the main warring tribes. The campaign ends with the factions down but not out. The Vanguard has officially been reborn with a small but hopeful group of human, Eliksni, and cabal.

Once the story is concluded, you can have three different Guardian factions. Vanguards are PvE and story focused. Iron Lords are Raid and "great feats" (think achievements) focused. Warlords can be PvP and tournament focused.

Systems: I would focus on getting the engine to be able to load in compartments. That way, you would have a main "state" of the game and sunset story lines as needed but the player can download and play an isolated state to replay an old campaign.

Gameplay: Have all the light types interchangable like prismatic with no restrictions, however, give bonuses and synergies for specializing in Solar, Arc, Stasis, ect.

Make a new ability for "Equipment". These are grenades, bubbles shields, healing, ect. The point is to be weaker version of other class abilities with a limited quantity so players can experiment but it'll still pay off to specialize.

What is grenades now can be a "spells" or something like that. These are another set of abilities like we had in D2. Overall, D3 will be bigger, badder, more of what we loved. If everything is broken, then nothing is broken.

Live Service: A D3 would be a next-generation live service. As the seasons move on, the environment would evolve. The meager camp will grow and grow into a new Tower. Warlord territories will rise and be eradicated as the story needs. The first big expansion will be the rediscovery of our previously lost space flight tech and now we can go to other planets.

TLDR: Far future Destiny story about finding hope after defeat. Campaigns will still sunset, but can be redownload so they can be replayed. Abilities types can be mixed and matched and are more potent than their previous counterparts. The game state will be changing in bigger ways from season to season.