My suggestion would be the following:
A quest that can't be obtained, could be given by a GM.

Conditions:
- The quest is already reported in the bugtracker, but it's pending to solve it since year ago or more than 6 months.
- You have done the previous quest.
- The quest must work.

Example:
In Outland's realm, Nexus-Prince Shaffar's Personal Chamber's quest can't be obtained (https://www.wowhead.com/quest=10981/...rsonal-chamber), when you reach exalted with Consortium's rep, although you should be able to get it being exalted too. It's already reported in the bugtracker 2 years ago (https://www.warmane.com/bugtracker/report/51241). This quest works, but it just turns invisible or dissappears being exalted from the NPC, so, it could be a temporary solution for similar cases like this one that a GM gives you the quest to can continue with that.