With the current design philosophy of ARR, a evasion based tank would break the game. Let's use the ever present twintania and extreme modes for examples. Firstly, we need to come to a understanding about evasion tank survivability. That is, for any semblance of balance, it's primary mitigation comes through evasion; meaning that it can't really take a hard hit. A obvious, but needed statement.

Now, for extreme modes, or turn 2 of coil. I cite stacking debuffs. The ability to completely mitigate these raid mechanics through dodging, would completely void PLD and WAR in these encounter. In the case of twintania, the ability to evade Death Sentence would once again, break the encounter to the evasion tanks favor.

Well, how do you fix such an obvious overpowered job? Well, for that to happen and not break the mechanics, would mean that DS or incinerate, or MB, or whatever would have to ALWAYS hit. meaning the evasion tanks primary means of mitigation is now void. Remember how evasion tanks can't really take strong hits, like death sentence? It would be underpowered in this case, if not completely unviable. This is only taking into account certain factors as well, for example, what about aoe damage? Now, I'm not saying a evasion tank isn't possible, but would require something really complex, such as tiered levels of evasion, you can't simply evade an attack but you can evade most of an attack. Which would be similar to a PLD, only with a better shield and fewer cooldowns. Of course, that's just a simple muse on my behalf and the actual viability of such a system is suspect.

Point is, I extremely doubt an evasion tank is ever going to happen, and if it does, it'll be nothing like what most people will expect or want.