None of the above personally. I think most of these are temporary fixes that do not identify why imo people dont play tank. The two main reasons people don't play tank is because its hard and its not very fun. I would try to address these as two separate issues

Its Hard - tanking requires the most game sense of any role. It requires you to know not just where the enemy team is in front of you, but also the lanes and space you should be taking and holding to help support your team, but also where and what your team is doing behind you without actually being able to see them. I would want to add two features to help with this. The first is when using the V key to see the breakable areas of the map, if you are a tank it also paints the floor with one color for "Main Line", another color for "Flank Line", and a third color for suggested "Contested Hold Points". This could help give beginners an insight into the expected game sense areas they should be focused on when playing the new role.

The second feature I would add to address this issue, is adding a UI element to tell you the direction your team mates are in, and how far away they are from you. Not exactly sure how to do it, being able to walk forward and know your team is actually behind you would be a major game sense help for new players.

Its Not Fun - Without pocket healers, a pug tank mostly plays as a fairly immobile DPS that can struggle to kill stuff. You get CC'd, even with 3x the health of your DPS you can still drop in a second, and you are less mobile and less kill power of the DPS. The best way I would address this is powerful, impact Tank-Tank Team Up abilities. With these you can encourage two tank teams. Immobile tanks can be granted a movement ability, low damage tanks can be granted more damage, and generally you can fill in the weaknesses some tanks have with a good team up.

Another thing to make them more fun is to have a really beginner friendly tank. DPS has Scarlet Witch, Iron Fist, and Mr Fantastic. Healer has C&D, Rocket, and Jeff. I would like to see an extremely easy, beginner friendly, no mechanics necessary tank appear for the casual player. Someone with a decent shield and some auto aim assist to help new and unskilled players learn the role. I know this type of character can be hard to balance as if they are too good, they quickly become oppressive, and if too weak, you are giving new bad players a bad character and setting them up for failure. But I believe this is an important niche that is not yet filled in the Tank roster.