How hard is to develop an independent (i.e, not based on another) WOW server emulator?
Over the last years, decompilers have appeared that can speed up the development, as one could decompile the client to figure out the protocol, but some things are heavily obfuscated (like the Battle.net protocol, apparently). Also Blizzard has made it hard to develop a emulator for expansions after WotLK, e.g by introducing a phasing system in Cataclysm and in WoD, making a Battle.net server mandatory, introducing a new container format (CASC) and reworking the phasing system (removing the phase names).
Furthermore, all emulated servers must be populated with maps, DBCs, and others to prevent the world from being “empty”.