One - I want to see legendaries that you make character builds around. An example would be Earth Shifter in Diablo 2.
Earth Shifter provided +7 to Elemental Skills for a Druid (which was MASSIVE), had a chance on hit to cast a high level Fissure (which synergized with any skill points you placed that affected Fissure), and had very powerful melee damage and stats (ridiculously high damage, increased attack speed, increased cast speed, chance of crushing blow) which made it a very effective melee weapon. It was also a two-handed mace, which the Druid class received an attack speed bonus using. This item basically screamed "Armageddon Werewolf" as Armageddon was the only Elemental spell a Druid could cast while shapeshifted. While the item wasn't very useful to anyone but that specific spec, that's what made it awesome.... highly specialized items make the game and the item hunt a lot more fun, and a really cool and specialized item can convince players to do something they want instead of following someone else's mold.
Where Diablo 3 made an attempt at something like this would be with http://us.battle.net/d3/en/item/frostburn-gauntlets
Unfortunately, those are a terrible item. Now that's not because there's anything wrong with cold damage skills, but rather because the item's other stats are lackluster at best, and simply getting a normal pair of gloves with decent stats (intelligence, crit chance, crit damage, attack speed) will provide a very similar boost to your cold skills and everything else
meaning that Frostburn Gauntlets are a terrible item. You'll get a 25% boost or whatever to your cold skills simply by using a normal pair of gloves and not gimp anything else in the process.
I've said before that the randomness of Diablo 3's legendary items isn't all that fun. That doesn't mean they can't be highly specialized. Highly specialized legendary items are good. A few have shown up (new Stone of Jordan) but there aren't nearly enough. Highly random legendary items that can be absolutely terrible for everyone are not fun. Replace the health globe stuff on Frostburn Gauntlets with Intelligence or Dexterity and crit chance, and a bonus to Ray of Frost, Hydra (for Frost Hydra), Meteor (for Comet) if Intelligence rolls, or Elemental Arrow (for Frost Arrow) if Dexterity rolls, and you have a really cool item (on two levels) that some people wanting to try something new would love to pick up.
As it is right now, all the same stats are useful to everyone (excluding primary stats) and that's incredibly boring. Specialized items that are very good for a select few add a lot of flavor and fun. I also think it would help build diversity a lot.
Two - I very strongly feel that a long-term investment into your character's build at end game is something Diablo 3 is sorely lacking. Character building, something to give your character identity through time investment. As many are aware, the Mystic (Karina in act 1 I think?) was removed as an artisan that basically acted like the Diablo version of a WoW enchanter.
I think a very interesting way to make use of her would be to use her to craft "charms" that can be used in new equipment slots. The only way to get these charms would be to craft them and have her upgrade them - no charm drops.
Basically, your character could gain 3 new equipment slots, and in those equipment slots you could craft a small, medium, and a large charm. These charms would have "themed" bonuses.
Examples might be, as a Wizard (easiest for me as I play one):
Large charms can be crafted and upgraded to augment specific spell schools. You could have a charm that increases damage done with Fire, Lightning or Cold skills (or other interesting element-based or themed bonuses). A Witch Doctor might have charms that improve pet, Poison, Fire etc. Melee ones might need to be based on weapon types or something.
Medium charms augment defensive abilities of your choice, or possibly provide passive defenses if you choose that.
Small charms provide a modest bonus to a second skill type, say if I wanted to use Fire Bolts primary and Ray of Frost as my AP spender (yes I know that's terrible) I could have a Frost large charm and a Fire small charm. Small charms could also use the already existing skill mods and give players the option to choose the one that suits their favorite skill.
At any rate, the idea is that crafting charms of your choice would be fairly simple at first, so you could get a reasonable bonus to whatever you wanted easily, and switch them out easily, but truly maximizing your charm and therefore your character (done by "upgrading" an existing charm with gold/material resources rather than creating a new one) would take a great deal of time and effort.
I think these charms could consume gems and crafting materials to provide the bonuses to the areas you like, providing uses for these things that currently have none while appealing to the Diablo veterans who enjoyed long term character builds and investments with some form of permanence.
This way, players can try out all their skills as they get them while leveling as the skill system would remain very flexible, so at end game they can know what they like and start to specialize but still have the option of switching if they choose to.