This might sound like a crazy idea, but it's something I've always wanted in an MMO: a caster that tanks by summoning a bad-ass companion, while the caster stands at the back and uses spells that help the companion do tank stuff. This includes weaker shields/heals that only affect the companion. The caster would also have some damaging spells for them to use sometimes, as well as certain cooldowns that buff the damage of the companion for a while. Plus a skill that transfers all enmity to the companion.

The caster and the companion's HP percentages are linked. So healing one would heal the other, and damage is shared between the two. However, the companion takes less damage when being hit. So a boss might hit you for 15,000 with an auto, you'll both take 7,500 damage. But if the boss hits your companion instead, it'll only do 10,000 and you'll both just lose 5,000 HP each. You'd have a resource-based spell on a low cooldown that makes you immune to damage (but not your pet's damage) for 2-3 seconds, so that you can avoid getting hit by party-wide AoE attacks, since if you and your pet get hit, if basically means you're taking twice the damage.

Upon the death of the companion, the caster survives with 1 HP and all enmity is transfered to them, but they become invulnerable for 5 seconds. The caster has a skill on a 4-5 minute cooldown that let's them instant-cast another companion that immediately regains enmity. This serves as their invulnerability mechanic.

The shields and heals from the caster wouldn't be enough to keep the companion alive. You'd need healers to help out. Whether the caster chooses to heal/shield the companion as well, or use their DPS spells, would be the difference between tank stance and DPS stance. The spells would be the shared -
under one stance, they're target-based damage spells/DoTs. Under another, they become heals/shields that you can only use on your companion and generate enmity for your companion. You'd have a skill with a moderate cooldown that transfers enmity to your companion. As well as cooldowns that become buffs to your companion depending on your stance as well.

Playing this would require a lot of coordination, as you would be having to position your companion appropriately to make sure the boss is facing the right way. You'd also have to make sure you don't get hit by AoE attacks. I don't see it being easy, but I sure as hell would have a blast playing something like this.