Borderlands 3 Story should go like that:

In the beginning we can choose from few new characters but why i will explain later, they are vault hunters.

Tannis will give us first quest after landing on pandora and this quest should inform us about some new artifact and old friend of her that is in need. (Lilith)

Quest for Lilith is to capture one of old but still functional Dahl spaceship located in the far distance of pandora.
Ship has still intact warp( FTL) drive and is needed to jump to new vault on other planet.
This vault contain artifact that with combined powers of at least 3 sirens (Lilith, Maya and Tannis) can warp through time.

There entire spaceship is warped back to save Roland, Scooter and every one that didn’t deserve to die.
But first jump ended in wrong timeline and in the beginning of pandora still populated with guardians, just starting to construct vaults. After jump, artifact discharge itself and teleport with Lilith somewhere on pandora surface.(possibility of creating Eridium pocket under surface of pandora that will growth combined with Lilith power and some artifact particles and crystallize through ages)

Until finding Lilith player need to fight guardians, Lilith create alliance with guardians after informing them about situation. Guardians seek for help with putting destroyer in the vault prison. This Destroyer is far more danger than this one released from vault because it have full potential after making his way to pandora. All characters need to cooperate to fulfill this quest. To preserve timeline Guardians put them to sleep leaving behind just few of them (the missing of guardians mistery)

After finishing Old Pandora quests sirens try to jump second time, this jump ended in the beginning of pandora colonization. There we find baby scooter and baby Ellie, we need to create path that put them to their way of being them in the future. Also player need to sabotage Dahl mining operations to get corporation off the pandora so Panora will became what it is in future. This operations need to be stealth and all border 1 and border 2 characters can’t be seen by npc’s.

Its not all scenario but the end of it should look like that: Players could rescue Roland after final scene and revive him using some Guardian piece of technology, and bring him to the ship Roland will continue his timeline with characters in future, also scooter will be teleported to ship by Lilith this quest need to be done in precise time.

Also player could not interfere with timeline and can’t kill Jack before he kill Roland and can’t do anything to change this what needed to be. After returning to timeline player should noticed some little changes like, skinny Ellie, Marcus without Mustache, Zed having medical license etc.