So Path of Exile is one of the few games I've played where your build in regards to your defense is just as important, if not MORE important, then your actual offense. While the game isn't perfect, I think it has a really good set of core principles.
To summarize, this game has 3 basic stats which kind of guide all the characters- Strength, Dexterity, and Intelligence. All the character classes are either “pure stat” in one of the stats or are “mixed stats” and have two. So for example, there is the pure Strength Marauder, who is a generic big honorable warrior dude, and there is the Strength/Intelligence Templar, who is a strong priestly crusader who can also do magic stuff. Every class actually has the access to the same abilities (skill gems, which are socketed into equipment), the difference between classes is there starting location on the skill tree, some base stats, and their ascendancy class (subclass). The exception is the Scion who is all three stats at once and starts at the center of the passive tree and only has one ascendancy class, but it can kind of steal abilities from all the other classes.
Now if you're seeing a pattern here it very firmly falls into the sort of Fighter, Rogue, Mage sort of thing that I am fond of. I like it in video games but also in tabletop games too. But each one has a different method of avoiding damage that fits with the theme of each stat- these are the Defenses.
Strength is Armour- which reduces damage taken.
Dexterity is Evasion- which makes it harder for enemies to hit. Also reduces chance of taking critical strikes.
Intelligence is Energy Shield- a second life bar over the first which recharges quickly outside of combat, but cannot be restored with potions.
As you can see, this triangle system is really interesting and sets up a lot of nice ideas, and even different gameplay elements for each character. Armour reducing damage from Armor is totally fine, very basic- they just run up next to enemies and attack them up close. Since they can 'tank' the most hits it fits the character, but the problem here is they may lack depth. One way to solve this is because they ARE taking more damage more often even if it is highly reduced, having rapid use block or damage absorption abilities and make them rely on healing a bunch is a good way to pull this off. To be fair, these are actually things the Path of Exile developers are doing in recent years to make the game more deep, and I appreciate it and think it's good, though there is still a noticeable bit of carelessness that creeps into your play as a melee character. I'm fairly certain that the reason Cyclone is such a popular skill to build around league after league is because it just lets you turn your brain off- and trust me, I love Cyclone, but I don't really think that's the kind of play experience you want when you're playing an ARPG.
Evasive characters don't get hit often, but when they do it can be pretty dangerous- though it does reduce the chance of getting crit (enemies need to roll to hit you twice to confirm a crit I think). However, this means that all the damage you're taking is just a dice roll, there's little game play difference, which is the core of the issue here. I should also mention that some games, like the other PoE (Pillars of Eternity) has 'glancing blows' which I think is another good way to make Evasion more effective. Stagger the kinds of hits/damage/crits into different categories- like miss, glance, hit, crit, etc. and evasion gives you a better or worse chance to evade these.
Finally, Energy Shield fits perfectly for a Mage type character- staying far away from enemies and spamming spells. They need to run away to recharge their energy, and energy shield also absorbs damage from elemental attacks that the other defense types; they only block physical damage/attacks.
Now while D&D uses Armor as Damage Avoidance in that system, we already kind of have this kind of set up in a lot of D&D/OSR/DIY games already. Fighters have the best armor so (even without Damge Reduction armor) they take the least damage from direct combat hits. Rogues on the other hand aren't as good as combat but tend to have the best saves and have stealth, which means they avoid those types of dangers more often, and Magic-Users have spells that buff and protect themselves and others, which could be thought up as a type of defense that the others don't have. For example, using a wind spell to clear away toxic gas or turning undead to avoid level drain are defenses the other characters don't have access to at all.
Suggestions for Evasion in Video Games
This is a video game idea from someone who doesn't make video games, but I've thought about it a lot. Basically, my biggest complaint for how the above system works is too much randomness, especially for Evasive characters. Personally I'm a big fan of randomness, but it doesn't feel as good to build out an evasive character, especially that there are other ancillary systems like Dodge (which has nothing to do with evasion whatsoever) and Block (which is good, but anyone can do it and isn't really tied to anything in particular.) It also grants little defense from spells, though it may grant more defense against elemental damage since armor just absorbs physical, I think, so avoiding an elemental attack is a slight edge in defense here.
So here's my suggestion- Evasive characters are meant to be fast on their feet, right? Well PoE already kind of does this with most of the Dex characters having nodes or skills that are all about moving quickly already. So you're kind of expected to dodge more as an evasive character. This works, but it has nothing to do with the actual Evasion defense or how much evasion you build! So the idea is to combine the mechanics a bit; physically change the area of effect or damage radius of enemy attacks in the game. Now I know that may sound crazy, but consider that in most ARPGs or games with movement and combat; enemies have certain telegraphs or big damage circles that appear that you have to move out of. Stacking high levels of Evasion on your character could have the extra effect of literally shrinking these circles. It would be a client side sort of thing- you see a smaller circle then another character would against the same boss. This is already in the game to an extent, as maps or zones can be modified to have greater enemy Area of Effect, so it's not some pre-programmed value for every attack.
The biggest problem I see with this is two fold. The first is what happens in multiplayer. This is the beauty of a client-based online game like this with different screens; every player sees exactly what their character would, so enemy AoE are smaller for an evasive fast Ranger, because she knows how to skirt the edges of an enemy's blow to best avoid getting hurt, where as the notably more clumsy Witch could not. However, you might see another player in your game session walking into enemy Area of Effect circles and not taking damage when they explode- causing you to think the game is glitched or they are hacking! I have a solution for this; make it so if a player on your screen is in an AoE you can see, but is too small for them on their screen, little 'action lines' appear around their character model as they walk. Then, when the boss smashes down and they are in the AoE for an ally, but not for themselves, they won't take the damage- and you can play an extra client-side only animation of them rapidly dodging away, or like a cool flashy afterimage glow effect that makes it appear as though they just dodged the attack in the nick of time. I think this solution would be extremely cool in any ARPG, not just Path of Exile, as evasive characters would have different playstyles to everyone else- based more on dodging and weaving through enemies.
The second is cheesy shit, like switching to evasive gear right before or during an enemy's wind up animation, to help make it easier to avoid. I don't have a good solution for this- instead maybe the evasion mechanic doesn't actually shrink the circles, but just makes them do less damage farther away from the center. This makes evasion more important, though it doesn't directly solve the above issue. Still, I think this idea is unique and has merit, and is a unique kind of mechanic that only works and is uniquely suited to the video game genre as a whole.