Items: Legendaries, Artifacts, Customization

Artifacts

The Artifact weapons are in my opinion one of the best item-related new concepts added to the game - and one that should be kept in one way or another, possibly with a few adjustments.

• Visually, there's no real problem ever since transmogrification was introduced (or rather: once transmogrification enabled the use of the skins of different weapon categories). Even if you don't like a weapon's look, pick one from another item you like better.
•  The relic system was probably what made the system so good: instead of being utterly dependant on killing the final boss in the newest raid in order to get a really decent weapon, the relics enabled progress outside of specific encounters and in much smaller steps, which made improving your most important item much more rewarding. The fact that the artifact weapon usually was one of the most advanced items from a character's gear was pretty sweet as well for psychological reasons (it feels pathetic to hang around with a level 700 item if you're decked out in ilvl 720 gear).
• The leveling of the artifact weapon on the other hand wasn't such a good idea - it either forces a player into sticking to a particular spec because leveling another one takes too long or, if it doesn't, the mechanic is mostly redundant anyway. Also, tying character progression to an item begs the question why it wasn't tied to the character instead.

The following suggestions for the weapon system: While the artifact leveling mechanic is removed, a modified relic system for weapon upgrades on the other hand stays in place, and works as follows:

•  An artifact has major relics, minor relics, and possibly other things to introduce additional options for weapon customization, like f.ex. supplementary relics or runestones.
• A Relic's most important trait is that it improves a weapon's ilvl, which means it's essentially unchanged. Major relics give a big boost, minor relics give a smaller boost (or none at all).
• The second trait of major relics works similarly to an affix on a normal item, as it governs the distribution of the secondary stats, with the stat total depending on the ilvl of the relic. Example: if an ilvl 900 base weapon had three specific relics (ilvl+80, crit/mastery, ilvl+60 crit, ilvl+60 haste/mastery), then the final distribution would be 50% crit, 35% mastery, 15% haste from the stat total of an ilvl 1100 weapon.
• The second trait of minor relics is in a way the replacement for the artifact trait bonus. Instead of this, the relic offers a bonus to a skill instead (there are two options to handle this: either the bonus is moderate, but one can't use multiple relics of the same type; or the bonus is small, but it's possible to stack similar relics). This trait is outsourced to the minor relics so people who want their weapon tailormade for specific playing style aren't forced to decide between a high ilvl relic with a useless bonus and a low ilvl relic that complements their playing style.
• Supplementary relics would either add a big chunk of tertiary stats or a flat bonus (like, say, +10% running speed, freely stacking with all other bonuses to running speed)
• Runestones could offer procs (like applying a flame burst, slowing down or rooting an opponent, applying a bleeding dot, lifeleech etc.) and work not unlike weapon enchantments. Or they could offer on-use-abilities like the artifacts in Legion did (the difference being that these abilities shouldn't be vital to a specific specialization).

A fully upgraded Artifact weapon.

Legendaries

Legendaries as such aren't necessarily a bad idea since they complement a character's itemization by adding vital items which are on par with item sets. The problem is mostly the fact that since Legion, they are (a) poorly balanced and (b) completely random. This is not how things should be in a game where gear rules supreme.

Tying legendaries to quest chains as it was done in MoP and WoD was tantamount to introducing gated content for alts, and as a consequence was done away with (rightly so). However, the legendary flood in Legion wasn't exactly the best way to remedy this problem. If legendaries stay in the game, acquiring them should be a lot more predictable.

•  Legendaries can be crafted, similar to the broken shore quest.
• Legendaries can be acquired via (class) quest chains and players are free to chose them. These quest chains however should be far shorter than the ones in MoP or WoD (not unreasonable considering that there are far more of them).
• Bosses drop legendary tokens which can be traded in for legendaries by remote and difficult-to-access vendors. 

Item upgrades and customizations

The upgrade system in Legion (with ultra-titanforged items) was wayyyy over the top and actually hurt the game more than it helped since it devalued items gained from more difficult content and forced people to run content below their skill level in the hope of getting lucky procs.

• Reforging should be brought back.
• Simple titanforged should be the hard ceiling for ilvl gains. An item should not be better than one found in the next raid difficulty tier. 
• Gem socket numbers should vary again - up to 3 for helmet/chest/legs/trinkets, 2 for shoulders/feet, 1 for the rest (or whatever combination is feasible - it's not even carved in stone that 3 is the ceiling). To account for this adjustment, gems should be nerfed to some extent. This is to make sure sockets are roughly on par with an ilvl increase instead of being good for minor items (like gloves) but bad for major ones (like the chest). 
• Instead of getting the stats at random at the time of the drop, the items can be upgraded afterwards: +5 ilvl are the baseline, and on top of that two of the following: (a) either another +5 ilvl (b) the full socket number (c) one tertiary stat (d) another tertiary stat. 
• Upgrading is enabled by a combination of raid and mythic drops, dungeon and scenario drops, valor, crafting materials, crafting and gold.
• Artifacts, if left in the game, shouldn't get any tertiary stats or gem sockets. They're already very customizable the way they are. Legendaries on the other hand might warrant some customization - the difference being that the scope of the modifications is far larger (like, say, getting legendary gems for the slots, having the ilvl bonus doubled, and getting 4 instead of 2 possible additional upgrades).