Added (2018-11-14, 3:34 PM)
I think most players can agree that Defender as he is now is insanely above the curve for how powerful a hero should be. He pretty much uas no weaknesses, his early, mid and lategame are all exceptional and he is not reliant on specific drops and does not need to farm stats zo be effective like Assassin, Abo or Unholy Knight do. Simply put, he's overpowered. So let's think of some way to bring him down to earth.
Sacrifice is arguably the most powerful ability in the game. It deals insane damage in a huge area and stuns for up to 10 seconds with a decently short cooldown. *If there was a hero who had only Sacrifice and no other abilities I would still pick this hero, that's how good this one ability is*. I can't even point at any one nerf it needs, the damage, the area, the stun duration, the cooldown, all of them are too good IMO. It costs HP to use, but by the time the stun ends you have regenerated more HP than you lost so it's not really a choice there (more a good thing cause it lets you be damaged to use Revenge for a second).
2. Make his abilities stack with his real armor and reduce effect
We currently use Warcraft 1.28.5 but for future versions maybe we should switch to higher versions because Warcraft 1.29 added a simple function to get the total armor of a unit, this means Defender's abilities Shield Slam and Divine Toughness can actually use his real armor instead of only his base and hero ability bonus armor. Now as they are Shield Slam and Divine Toughness would be extremely overpowered, but with their effect reduced to 1/2 or even 1/3 of current they would be much more balanced - this way Defender also benefits from getting items that boost his armor further, which would be great. And it would even out the curve and make the abilities similarly strong or even stronger than now in the lategame, but remove much of their power in the midgame.
Another idea for Divine Toughness: make it give only HP instead of Strength, this way it still gives defensive stats but can't be abused with God Dark Blast or Unholy Crystal.
3. Mega Heal
Mega Heal is a misleading name because it not only heals but also does massive damage and gives a permanent bonus, making this kind of a super all in one ability. It's much more powerful than most other heal and damage abilities in the game. I think the damage part should be removed entirely, it has no business dealing damage, and the amount of HP it heals should probably be lower as well, it's almost always a full heal right now.
4. Damage Breaker
I think this ability is good enough without also dealing % of total HP in damage, maybe make the effect not removable instead. Why total HP % damage this is just insanely powerful and can demonstrably be used to kill bosses like Titan solo.
This ability is mostly fine but I think it is too easy to be too powerful with it - how about halving both damage and cooldown? Then you at least need to spend more time being damaged and using it.
6. Not allow reskill?
Not sure if this is a good idea but maybe make it so Defender (and Berserker) cannot choose different abilities after changing attack type? That would by itself already make them much less powerful as well, and force long term planning and specialization from the start.
These changes are the first that come to mind for fixing Defender and making him into what he should be: a hero good at defending. :P
Bonus: I feel Dark Mage Water Elemental Water Blow should be reduced to dealing 2 times total HP on damage instead of 4 times. If Defender is nerfed but Water Blow is not, Dark Mage will be the next dominating hero I think, though mainly for his early power instead of being good at, well, *everything*.