The Problem:
Some legendary aspects, and most unique effects are simply too powerful. It takes away item affixes from the equation, because if the effect is powerful enough you’re not gonna care about the affixes.
The same also goes with affixes when they’re strong enough to overcome Unique effects and Legendary Aspects such as Tyrael’s Might that has enough damage reduction to surpass two defensive aspects, or Fists of Fate that has enough Lucky Hit Chance to completely disregard its Unique Effect (like Andariel’s Rogue).
These two ‘systems’ are not harmonious with each other, there’s no synergy.
Just looking at Lightning Spear sorceress for an example. With 1h and off hand you’re getting more desirable stats, and I think most people agree that gameplay is often smoother and more enjoyable with faster weapons. But because their main damage comes from an Aspect you’ll benefit more from a 2H staff (with an implicit that does nothing for you) because it doubles the damage of the aspect.
Let’s look at build-enabling Uniques such as the Fireball staff and gloves. It’s not only adding a ton of flat damage, but also adding additional effects that further increases its damage. At most Fireball is put on par with many other builds, and that’s only for AoE (afaik boss dmg is still low). And what this tells me is that Fireball is just badly designed and balanced by itself, and needs ridiculous boosts to catch up to other builds, which end up further diluting the affix-pool and unique/aspect effects.
The Solution:
The solution is to remove Legendary Aspects and Unique effects from the items, and add in a separate UI where you can mix Legendary Aspects and Unique Effects without directly tying it to the items (let’s call it Legendary and Unique aspects).
The Aspects could drop on the ground, much like tempering manuals do. You would right click it, and it will be put in the codex and then only better versions of that aspect can drop. You could even separate the values by having it drop as Magic, Rare, Legendary just like the Tempering Manuals, rather than keep getting manuals drop until your aspect is at max value.
Uniques should still be designed to have unique-affixes. Meaning that Uniques can have affixes that is normally unavailable on certain slots, or affixes that otherwise don’t exist on any slots.
Whereas Legendaries should still have random affixes from the fixed pool of affixes. Maybe you can even bring back rares (in place of current legendaries) and do something cooler with legendaries.
Uniques should be more generic, and not so build-specific. Just go crazy with all kinds of interesting affixes you can give things that are useable for multiple builds and not just one of them. It will instantly give us more options, and we can truly min-max our builds by going after the exact kind of stats we want for our build.