If they make adds more dangerous, they would potentially need to change how the mob AI works when it spawns. Currently, when they spawn, there is a few seconds (sometimes less) before the mob picks a target and attacks. This attack cannot be stopped even if you manage to get to top enmity before the attack goes off, unless you stop it in some way, either via a stun or interrupt (if applicable).

Going onto crits, I could swear boss autos crit, but I could be mistaken. Nothing is dangerous enough to even have it be an issue, however I do know tank busters cannot crit at all. Regardless, this talk of critical hits makes me think of Shiva and her bow phase, where she has a massively high crit rate ( at least in EX, and I believe it is in unreal as well). Even though defensive cooldowns, they still hurt and you can tell some of the less knowledgeable healers are bit surprised by this.

Going with this, you could make boss phases where they have periods of increased damage, whether it be from a higher crit frequency or another way to increase damage, but still have the tank busters. Shiva EX/Unreal suffers slightly in that there are no tank busters to mitigate, so your defensive cooldowns are best used to mitigate the bow, however, if you add tank busters, you suddenly have to juggle the cooldowns between the 2. And as a 1:30am thought, this could then provide a requirement to tank swap. Not to ensure the tank doesn't die because of a debuff, but making your healers life alot easier.

Reading through the thread, you can tell most people do not care that enmity is easy to maintain, that was not the fun part of tanking. It was the positioning and mitigating of the boss to help the group. Mitigating can be changed as I suggested above, however, how can you sort out positioning? Again, I can turn to Shiva EX/Unreal as a good example. Ignoring the fact she doesn't like moving, there is in total 2 points in the fight where she auto positions herself. The first is the start of the add phase and after Diamond Dust where she spawns on the highest enmity tank. Because of this I can easily tell tanks who know about good positioning and those that do not. After Bow phase, most would agree it is beneficial to return her to the centre and face her North. This basically acts like a boss position reset, but it is carried out by the tank and not the boss themselves. More things like this would help to solidify the importance of proper positioning and could help to make tanking more interesting.