TL:DR - D4 legendary items seem cool so far. More item tiers could be added without cheesing it, Legendary - then SuperduperLegdendary. (Example item tiers below), Crafting could add runewords back, name the current D4 “Runewords” something else. Also Jewels! Could be crafted items that allow for neat itemization options via sockets. Take affixes off one item, put onto a jewel and socket into another item, 1 affix for blue Jewels, 2 for yellows Jewels, Mythic Jewels take the Legendary power. 
Legendary Items 
Current examples of D4 Legendary items are pretty neat. I like that they don’t seem to be going into the whole “10000000% Damage” direction. They actually change how you use the skill give you more build options, especially if they have a variety of legendary items per skill. Also legendary items that overall change something based on damage type could allow for more crossover between classes. One of the annoying things about D3 legendary items is that some of them with crossover were really generic and boring, not really “Build defining” had some staples like Nemesis Bracers. But overall not really something you’d use to build around.
Item Tiers 
Honestly, this is something I’d like to see the D4 team improve upon. Hopefully they show more in the “Endgame” update around December/January.
Normal-Magic-Rare-Legendary-Set-Unique. Seems underwhelming and I feel like this could be improved further. Just don’t add SuperDuperLegendary. Such a cheese way to put in more tiers. I liked when they had talked about the “Angelic and Demonic” power. But they inspired me to see these as Item tiers instead of what they were originally.
Demonic 
- Basically cursed items, Roll 3-4 Affixes that are boosted beyond normal means, but come with a negative drawback. Like “increase fire damage taken by 30%” or something. (I am not sure how hard it is to balance these but they are interesting none the less.) (could be balanced with a limit to 1 or 2. Don’t wanna be wearing too many curses the body can only handle so much.)
Angelic 
- These roll max sockets and have boosted innate stats. So if a Dagger always has 4% crit chance no matter what dagger you find, then that would be boosted on Angelic items, along with the base damage of the item. These would mainly be desired for custom items through the use of socket items or crafting bases.
Crafting 
Another thing I hope to see mentioned in the endgame update.
One idea I did have for Runewords was keep what they have now with the Condition and Effect thing but don’t call those Runewords. But allow for like a crafting option with actual Runewords.
Runes + crafting mats + Item Base + gold cost = Runeword item
Runeword Item - Unique items without a “power” I think runewords would have set affixes. But they could create unique affixes for these. Roughly anywhere between 5-7 affixes. These can also be something limited or Semi-limited to 1-3. This would bring back Runewords and allow them to do it in a more balanced way. While giving players more crafting options.
Every 10 points of Intelligence grants .2% Fire Res
Stuff like that.
Jewels 
These were always pretty neat in D2, but for D4 they would be neat as a crafting source to further increase the usefulness of lower tiers.
Blue item + Jewel + Jewel Casing + Crafting Mats = Socket-able jewel with 1 affix rolled from the item used. Value of affixes are random. 1 affix blue items would guarantee that affix, just not the value.
Yellow Item + Jewel + Jewel Casing + Crafting Mats = The jewel gains two affixes with random value rolls.
Legendary Item + Jewel + Jewel Casing + Crafting Mats = Mythic Jewel - The jewel gains the legendary power and can be socketed into any item. Even another legendary
This would allow for Yellow items to have a max of 7 affixes with 2 sockets and 2 yellow jewels! Or 6 + Legendary power! Magic items can have the highest rolls so some Magic items with a high roll and socket, could have legendary powers or 3 affixes with a yellow jewel. This creates an interesting situation for using jewels to make interesting itemization choices, especially at endgame.
