Blizzard has had ample time to come up with a fix for this problem. The "there are more DPS queuing than there are tanks and healers" is tired and outdated.
Remember (forgot which xpac) Blizzard said leveling as a straight healer would be possible and running solo scenarios for rewards as a healer would be a thing?
Why not implement a scenario for singular people without guilds, without the 20+ minutes of waiting AND then running said dungeon?
AI has worked fine in brawlers guild, that gives rewards. It was viable in Proving Grounds, we started the fights as whatever spec we wanted...
This is possible.