Tanks for a long time now have kind of taken a back seat in a lot of content to brutes. A well built brute can tank as well as tanks and do more damage, so people usually prefer them over tanks at end game especially. My idea for a change would be to how tanks handle aggro to increase their role as "Tank" while leaving brutes unchanged.
1) Increase tank specific aggro cap to roughly an extra mob in size. This would allow for them to grab everything and anything else someone accidentally pulls.
:Clarification: By "Mob" I mean the an entire extra group. Not a singular person.
2) Enemies taunted by the tank rush into melee regardless of their preferred range. You can already do this with pulling/Los but It'd give tanks a great utility over a brute in terms of tanking ability/mob positioning. This could probably just be done by making the -range on their passive taunt/taunt ability to be 100%.
Edit: People seem to be misunderstanding the purpose, this has little to do with tanking power and more tanking utility. These changes wouldn't make it any stronger, just add extra utility to the tank as a tank. No changes would be made to their HP or damage.