With D3 being online only (on PC, I think on console modding would be even harder because of their copy protection) and any addon/mod/macro being "illegal" (so never supported by blizzard, accounts will even get banned), people would have to write a server first that's compatible with the current (and upcoming) official D3, before being able to make their own xpac.
Apart from the huge amount of time it takes (there is no official info, everything has to be reversed engineered), having a server available online that is compatible with D3 will result in lawsuits (look at what they do with wow servers). Running a local server wouldn't work either, since it will make any social aspect (grifts, public parties, trading if it would be implemented, ...) obsolete.
So, the best solution would be to rewrite everything from scratch, without any of the official art or story (copyright again). And while doing that, why reproduce an old engine and the imbalanced mess D3 is, without any official link to the game, only to fix and expand it, if you can just make a new game from scratch without having to worry about anything?
Last point, blizzard is already working on something in the diablo universe (according to the job postings at least). I'm pretty sure, if your suggestion would ever be started, blizzard will be faster with the next diablo game. They have the money, experience, people (like the art team that's shared between projects), probably a huge part of the story (since there was a 2nd xpac planned, they must have had a big part of the story in mind when they made ROS), a huge codebase they can reuse from and update, ...
I don't see any way this could ever get launched, survive for a week and be better than what blizzard will come with.