It really doesn't matter how many new defensive cooldowns you design. The limiting factors are fight design and the existence of invulns.

Current fight design places all the mitigation emphasis on a few tankbusters that always happen at fixed timepoints. The problem with this design is that there are only two tank cooldowns: invuln and supercooldown. You invuln the first tankbuster. You pop all your remaining cooldowns on the second. Then you swap to let them recharge. In fact, most of the time, there are so few of these damage spikes that you can just invuln-swap-invuln-swap. There's no need to ration out your cooldowns, and there's no need to know how much damage you actually need to mitigate. Just press the button when the quicktime event happens.

There are plenty of other damage patterns out there that are more challenging. The very simplest one is to bring in an add that hits very hard, with unmarked cleaves that happen every so often, and has progressively increasing damage (i.e. damage stacks). These are more challenging defensively because you need to ration out your cooldowns, and you need to figure out how much damage each cleave actually does. Maybe at three stacks, only a 30% cooldown or two 20% cooldowns (net 36% DR) work, while anything less oneshots you. And because you know that specific cleave happens four autos after the stack buff increases, you can time your cooldowns such that it eats two cleaves instead of just one. This is the most basic way that you can make defensive cooldowns more interesting.

If you don't need to ration out cooldowns and you don't need to know how much damage each attack does, the whole thing is trivial.