FWIW, there is a long history of illegal private servers that offer Vanilla and specific xpac servers for WoW. They are especially popular with people that are into player versus player because people tend to have certain xpacs that they believe were more fun for a variety of specific reasons, mostly related to balance between different character classes.
Every expansion has a "best class", that's for sure. Even if you try to balance everything, that's impossible to achieve; every class has dozens of spells, talents, buffs... and then, some of those spells, talents and buffs can interact with other classes (especially when in party or raid) improving or nerfing them.
In that expansion I'm sure you're talking about, paladins are somewhat better, and shamans are worse. That's normal, and it's not a problem when you play in live servers, since they are making modifications to spells and everything constantly. But when you're stuck in some version, then, you're stuck with how classes work in that version forever. That makes the population lean towards one class.
As worded, I think it would. Every bit of the map geometry and audiovisual content in WoW is client-side.
Server only handles chat (via hybrid-ircd, I think?), and via the main daemon (which seems like it might have been Lua-based at one point, or at least link into it like the client does), state coordination including item looting, and creep behaviour. Creep/boss behaviour and looting have been studied closely enough by tactically-minded players to be effectively black-box reverse-engineered, and fully reimplementable to a high degree of fidelity and precision.
(I'm not going to do it, I quit a while ago, but I did make a pretty popular addon, and did look into things a bit.)
