I think the context was meant as in the story and world that blizzard crafted. I personally would never back up or support in any way ''custom expansions'' for the same reason. The difference between game developers with long history and bunch of neckbeards in their mancaves crafting expansion is huge.
Now granted,nobody made a request but just tried to feed his/her curiosity so why the lash out - no idea.
But that being said,even common sense would dictate that yes,if you wanted to,and would have the team to do it with the knowledge required,you prity much could craft your own warcraft. the question is how long it would take,how many developers with.....look,simply put,to do what the OP asks,you'd need a copy of Blizzard's team.
Unless you plan to use already available assets,in which case you'd still need a department full of coders,3D artists,story writers and a train's worth of other people of specific skill.