I had this idea awhile back, WoD got me to thinking about it with the garrison #failedpotential but in reality, retail is probably far too big to implement such a plan, however being back on Classic, it got me to thinking about it again, and I do believe a career mode would be possible here.
How would it work?
The world starts exactly as it does now, but when you kill a mob, that’s it. It never respawns*. Your mission would be to clear each zone of hostile mobs, and clean up, rebuild and restore the zones to their natural un-corrupted form.
*(mobs that spawn other mobs will continue to spawn mobs until they themselves are killed.)
Capital cities would remain MMO as they are now, so that AH, crafting, finding groups, forming guilds, etc, all that would stay the same, because dungeons and raids would remain the same. Once you clear a dungeon in career mode, it remains clear (forever) and is no longer an instanced area and becomes part of your career progress. Unless you select to reset the dungeon/raid because you still want/need gear from there.
Same would apply to open world areas. For example lets say you need help with Araj the Summoner or Hogger. You can /lfg in a capitol city (or perhaps some version of the lfg tool in retail), and everyone in the party will gain the progress. If you are helping someone in an area you have already cleared, then the mobs will be present and available to kill as long as someone in your party needs to kill them. When you drop party, you are back in your career area with that particular area clear.
This means you have the option to do as much solo, or as much together as you want. Except of course content that cannot be soloed.
There could also be a career mode option a) restore the world or b) corrupt the world. In the event of plan b obviously, all friendlies must be killed, and when reputation becomes unfriendly, they all become hostile. I haven’t thought out option b yet, but I would make it be more difficult than option a. Maybe capitol cities, once your reputation becomes hated, would become a dungeon.
In either case, if you fail to clear a dungeon/raid of all bosses by Tuesday, then the weekly reset results in all progress for the dungeon being lost.
Just a rough idea so far, but I for one would be up for the challenge of restoring all of Azeroth and ridding it of demons and scourge once and for all.
^^