I think you're going to find the best use of your time to be the following:
Do World Quests and gather Argunite on
Argus for gear to make everything else easier.
Do you Class Hall questline for your
class-specific Order Hall mount (and the Broken Shore questline to work towards it as well).
Work towards any
Artifact Appearances you want to collect before BFA (they won't go away except for the Mage Tower appearances, but you won't want to be going back to get them for a while once you're focused on new content).
If you're interested in story and more unusual questing, I highly recommend picking away at the Suramar and Argus lines, but they are
not going to net you any meaningful rewards.
Don't bother with upgrading your Class Hall followers beyond getting them to 110, or whatever you need to complete your Order Hall and Broken Shore questlines, and you won't get much reward for doing the storyline quests in the various zones unless you're actually interested in the stories themselves.
If you're interested in the four currently available Allied Races, focus on completing Suramar if you want to unlock Nightborne, Highmountain for, well, Highmountain, and Argus for both Void Elves and Lightforged. However, those are all going to be long slogs.