A layer virtualizes continuous open world regions (like Kalimdor or Eastern Kingdoms) that are part of the world space (a “server”).
If 2 players (grouped or not) walk from Orgrimmar to Gadgetzan, they will see each other the entire trip because the layer is the same. The environment is virtualized; mobs, ore nodes and quest clickies, stuff like that is covered by this.
They have some leeway in tuning the layer concept I’m sure. But they are still bound by certain resources they still need to access and integrate with to operate the overall server world, which go beyond the purpose of layering for populated environment separation.
Shared server databases where they login, or read server-wide info, or update player state are shared by all players in all layers have increased load and stress with each layer added to a server. Any resource that isn’t independently cloned to support a new layer is at risk of this.
If you have 10 layers, you don’t have 10 distinct databases; more likely you just have some table schemas extended with columns like LayerId, and everyone is still slapped into the same database. Because even though we see the virtualization as a separation, behind the scenes it is still just a features of a huge server they have to maintain.
So, adding more layers isn’t as encapsulated an effect as we wish it were, and certainly not an easily horizontally-scaled setting they can just dial up.
What they can do, and IMO is what they are doing now, is increasing the per-layer cap (not the number OF layers) since players are much more diffused throughout the continuous zones in a layer now that everyone isn’t level 1-5. At launch they had to fit 100% of the layer population into like 6 starter areas and later that day spilling into probably another 6-10 city and level 5-10 areas. Each day that passes, they have a smaller % of the total population in any one place, and a much higher % of used zones.
As more distribution across levels progresses, they could opt to make the per-layer cap even larger.
Queues will be gone after one week, if not before, due to this.