To fully understand the interplay, consider terrain. This includes the placement of hills and valleys, trees, buildings, and more. When hooking up the old terrain data files to the new modern game system we realized that the way the system looked at the shape of data was different. This resulted in the updated system and classic data not aligning, resulting in weird issues like Kolkar campfires underwater or burnt-out trees from the Cataclysm era appearing in the original world.
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.
Since WoW Classic server has been announced at Blizzcon 2017, there is not much info about the server revealed. It is known that the upcoming expansion Battle for Azeroth pre-order has been available at blizzard shop, and the system requirements of Mac and Windows has been unveiled Recently, Forbes has an interview with J. Allen Brack and Jeremy Feasel about the project and some of the vision that developers have.
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