Other DD jobs and "Non-Tank" Jobs should not be able to Tank Safely. That's the reason "Tanks" exist, otherwise they're just useless classes.
Multiple jobs should be able to tank effectively, but not as well and not while still being primarily DD at the same time. Being capable of doing it requires being able to survive though ones definition of "safely" may very. I remember the days when nobody would do anything without one specific job tanking and I dislike that scenario almost as much as my pld not being useful.

Multiple tanks = good.
Making the tank job the only tank in town = bad.
"The tank job" can't tank the latest content = super bad.
Please remember this dev team. :x

No matter what changes come there always seems to be at least 1 job left unwanted, and after a few updates all we've done is changed which is shunned. Maybe that will never stop...Ugh.

Going to be real interesting seeing how they intend to balance things without changing enmity or using damage nerfs or giving every DD an enmity douse like ability.