How to get people to tank!

[Zojha]

You can change the balance by incentivizing the role more...

Or you can change the content to accommodate the imbalance better.
Or you can make content that doesn't rely on the roles - it's not that hard, you only need to replace unavoidable damage with mechanics.

You could adjust dungeons automatically - a group has more than 1 healer? Mobs get a global debuff to their HP and buff to their damage. A group has no healers? Autoattacks/Surefire AoE get replaced with telegraphed AoEs, tethers, stack mechanics etc - you can randomize that even. Lack a tank? Global debuff to damage on mobs, buff to HP, offset by healer adjustments. That's all theoretically doable simply with a global script you can run over the instance after the group is assembled. And it probably would even give the content some replayability because now every run is a bit different.

Trying to get people to tank...and especially forcing people to tank in the way the OP suggests IMO doesn't lead anywhere.