This is not news to anyone by now. I propose the following solution:
Allow dps specs to have the option to queue as tanks. Give them increased threat, health and/or armor and a vengeance-like mechanic. All classes now have tools for survival and some form of AoE so tanking multiple targets shouldn't be a problem.
This is only for 5-man instances or scenarios maybe, but not for raids. Even low level instances too.
Maybe with a requirement similar to You are Doing it Wrong in Proving Grounds.
This is about 5-mans, not LFR
Bolded part please.