We have programs, that implements WoW protocol.
SimpleWoW/MangosClient from different developers (forks, Aokromes's fork is tested by me). But we need to use UTF-8 (or CP1251).
PseuWoW from different developers (forks). Compilation not working on my laptop.
Clientless (I forgot developer). Auth works, but not detects character(s).
And maybe other. It is open source. We can write simple GUI and use client parts from another open source games like Ryzom (another 3D MMORPG). Then, we can add latest parts, for example, jumps.
Tools for open MPQ/DBC/other Blizzard files are already developed. We can include this tools in client. Noggit (editor for WoW) is open source, too.