Unlocking the Nightborne is a fairly straightforward process and, if you’ve been playing Legion, you’ve probably done most of the work. For starters, you’ll need to complete the main Nightborne storyline in Legion in order to unlock the “Insurrection” achievement. Completing these quests will go a long way toward earning you the Exalted reputation you’ll need. If you’re not quite there, then you’ll need to grind out Suramar world quests, though it shouldn’t take too long.
Instead of sharding the zones, their plan is to shard the entire continent at launch. In Classic WoW, there are no dynamic spawns and questing in starting zones would quickly become annoying, so it's understandable that the game is all but fun when there are no mobs to kill, but layering will also cause you to see new players, every time you sign in to the game, which is a clear downside of the tech.
Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.