This is a proposal for a suggested WoW system for item drops that makes use of the work spent on creating the Artifacts of Legion.

Basic Premise: Lift the Artifact Traits from the Artifact Weapons, tie them to a Glyph-like structure called Runes, to allow them to be picked up as drops and applied directly to spells in the player spellbook as an alternative direction for advancement.

The different artifact traits as they exist are unlocked as part of the Artifact Weapons, but could be directly applied to a character because they're invisible Buffs, essentially. Passive Spells that do not show on the player-facing GUI outside of their purchase option within the artifacts themselves.

Using the Glyph Method, we could apply the different buffs directly to individual spells and abilities, with both differing degrees of power tied to difficulty of content that allocates the items and explicit exclusivity. For an example, let's take the Harpoon ability of the Survival Hunter and the two artifact traits directly related to it.

By creating separate Runes tied to Eagle's Bite (The moderate DoT applied to the target of your harpoon) and Terms of Engagement (which resets Harpoon's cooldown when you kill an enemy) we could allow only one or the other to be applied to the spell within the player's spellbook. This would give players the opportunity to select the specific traits that they explicitly prefer. Similarly Sharpened Fangs and Mongoose Fury would be mutually exclusive Runes that could be applied to Mongoose Bite.

By tying the different traits to different levels of content we could also incentivize players to create their own "Artifact Track" of different content they'd like to complete in order to get the full suite of Runes that they personally prefer.

As to ensuring that players continue to work with higher content difficulties, we can use the different levels of individual artifact traits and apply them to specific difficulties of content. For example, a player might gain the first tier of Sharpened Fang (Increases damage of Mongoose Bite by 3%) by completing a heroic version of a given dungeon, while the Tier 2 version (6% increased damage to Mongoose Bite) could require them to complete a mythic version of the same dungeon. While the Tier 3 and Tier 4 versions could be tied to Normal and Mythic raiding, respectively.

Now this does bloat the Drop Tables significantly, with 21 or more individual drops and up to 4 tiers of each version. So I would suggest that these drops either be guaranteed rewards from specific content, or have a separate drop chance in a manner similar to the Pantheon Trinkets of Argus.

We would still face the issue of players retaining this power at the end of a given expansion, as well. But so long as the Runes and their buffs were tied to expansion-specific gear, such as the Heart of Azeroth, the system could be wiped clean with each expansion, and new options could slowly find their way into circulation in future expansions, each tied to a similar system.

Alternatively, these runes could have an explicit decay rate measured in months of time, say 6 months, after which point they need to be renewed, but cannot be used by a player of (X+1) level where X is the maximum level of the given expansion.

By being individual buffs to individual spells, these increases on their own would not be as large a drop as, say, a new weapon for a class that relies on weapons to increase their overall damage output. And with exclusivity, a player might be able to either increase the healing rate of a given spell or reduce the duration, but not both. I believe these factors offset some of the power creep issue.