Set items are a bad idea as it pigeonholes playstyle. Generally Set Items fall into 1 of 2 categories: Absolutely mandatory and build defining - or - worthless. Having flexibility in how you gear and build is very important.
It should not ever be obvious which Aspects to take. There should always be more options available than you can conceivably build for. It is pretty smart that the way the Aspects work, certain Aspect types can only appear on certain gear slots also. Having categories of Aspects also you can pick between more Offensive, Defensive, Utility, or Mobility. Some builds will have all the power they need from Skills and Paragon and desperately be lacking Resource managaement or Defense.
Personally, I think they can introduce a Rune System in a couple of different ways also. The 1st being creating a Gear Slot specifically for a Rune, very similar to Grim Dawn. For those unfamiliar, it adds a Movement Skill that has stats, +Skill, +Stats, Lucky Hit, and Abilities. The Movement Skills are classified differently as Charge, Engage, Disengage, AoE etc. This effectively gives you an additional Aspect Slot, which the most obvious choice being the choice between Movement, Utility, and Defensive Aspects only. There would be unique specific Aspects that can only roll on the Rune.
The second method would be to basically introduce a modified Gem system from Lost Ark. Instead of Gems, they are Runes. You would be given a certain amount of Slots and from there, you can build different Runewords. Some Runewords may require perhaps only 2 Runes, where others may require more or even the max slots. The Runes by themselves would provide different bonuses unique to the Rune. This basically replicates the Runeword system from D2, just not on Gear requiring Sockets and transforming your gear.
The twist to the Runeword system is that the Runes would provide Unique benefits NOT available in current itemization typically rather than it be basically a replica extra layer to Aspects. This can be anywhere from granting an Extra Skill slot for Runeword Abitilities, to Magic Find, Gold Find, Vendor Prices, Gamble Chance, Highlighting Enemies through walls, Weak Point exposure, Stagger Build, Stagger Damage multiplier, Passive Resource Regeneration, Skill Procs, etc. There is a lot of potential in a system like this.