Before starting out, I want to thank everyone who participated in suggesting new ideas (even the ones whose idea was not taken into account).
All of this took a lot of effort, so please, don't flame and be constructive if you'd like to comment and remember that my english is not perfect so don't insult me if some sentences are stupid and/or not very understandable.
Disclaimer: this list doesn't include (just for a matter of "too much popularity" or they've already been suggested) ideas like: add more random events, add an alchemist to create different type of potions/dyes, ban bots, add character customization (changing hari style, color etc.), add an NPC for in-game auctions, add a mobile and/or via web application for auctions, add a DotA scenario!, add an option to "nopickup" grey/white items, add more mobs!, add more active and passive ability slots, improve the chat and the UI (resizable, movable, etc.), increase inventory and stash sizes, add duels, make zones really random, bring the real Diablo music back.
With that said, here we go:
- Add a cooldown viewer that allows you to see, ontop of the icon of an ability, how many seconds remains until it is available again. Check/uncheck through the options menu
- Add the possibility to see the % of durability of the whole equip
- Add an option to be able to see the values on life, resource globes
- Add more details on the profile of every character (deaths, number of Diablo kills, etc)
- Give artisans the capability to "see" your stash, so you don't have to go back to it when you forget to bring with you your tomes or mats etc
- To give some utility to grey/white items, allow them to have sockets, and the number of sockets will be higher than a magic item (e.g. a white helm can have 2 sockets, a grey shield can have 3 sockets etc)
- Add a "safe-zone": when a character die and ressurect allow that he/she remains invulnerable until an action is done. This is to solve the "slow zone-loading for older PCs" problem. (You know.. when other people pull elites near an entrance and you find your character dead again because you PCs hasn't finished to load the zone in time for you to react)
- Add the "reforge" feature (the same in WoW): add an NPC that allows you to lower a stat and increase another one on an item for a price. To unlock new "reforge recipes" make them drop from mobs or give the NPC some quests that makes you unlock them.
- Make achievement points spendable: you can buy pets (gold companion ftw) or things for aesthetics (sigils etc)
- Make diaries/letters/journals that you find across the game bind to account, so you don't have to look for them everytime with a different character
- Add the Uber Inferno difficulty that differs from Inferno only for the first 2 acts. Act 1 and 2 will have the same level of difficulty of act 3 and 4 Inferno (same % of drops as well) so players can farm the area/act they like the most
- Add an option to convert the normal font into "Diablo font"
- Some magic properties are not easy to understand what they do so add a window that explains you what a magic property does when you mouseover it
- Add an in-game "bug-report application"
- Add the possibility to see abilities damage like it was in Diablo 2 maybe by modifying the tooltips of all abilities to be dynamic. In this way, instead of showing a fixed percetange of damage, they will show the real damage an ability will do
- Add Artifact items: these are legendary items that gain experience and improve over time (there can be low level artifacts - in this case they would improve together with your character - and end-game artifacts - in this case there would be something else to do other than farming when you reach the max level)
- Change how statistics work based on the class you are playing. With this, secondary stats would be reconsidered and revaluated. Eg. http://us.battle.net/d3/en/forum/topic/5959736506?page=6#114
- Add an option to turn on/off the screen-shake-effect
- Make health globes impossible to be picked up if the character is at full life
- Update the hitbox so your character won't get hit when you are far away from a mob or an arrow etc
- NEW - Add an option to change the color of the mouse cursor
- Add a tutorial to the game where you can learn how to use your character, how game mechanics work and most important of all you can learn the background story of your character (e.g. As a demon hunter you will see your family get killed by the demons and then you will be trained by your "masters" to become a demon hunter - maybe this can be implemented in an expansion... I think it's not possible with just a patch). The tutorial is skippable of course.
- Add the possibility to customize the name of a crafted item, maybe with a quest.
- Add the possibility to change the aspect (aesthetic) of an item (not only the color!)
- Allow the selection of non-standard parameters for certain kind of items (e.g. % block on helms or rings etc.)
- Add a filter to select a range of prices (min-max)
- Add a new detailed section on the Auction House window where you can follow the state of your offers (the current one is poor)
- Add a "refine search" function: when you get the list of items from your first search allow to set the filters again so you can search for items in that list that match the parameters you set the second time
- Add filters with conditions, exclusions and inclusions so you can search an item with these parameters for example: 1h weapon, with dexterity > 100, with vitality, without strenght, without intellect, with life on hit
- Add the possibility to select multiple "type of quality" for items: so if I'm interested in rares and legendaries I can check both of these. Like now I can set only one quality type (e.g. I set "rare" on the search parameters and I will get only rares)
- Let the player know when you succesfully won an acution or sold an item etc in the game chat (like in WoW)
- Allow players to sell unsold items from the "completed tab" without requiring you to send those items to the stash.
- Let us see the battletag of the players who won our auctions
- Add the possibility to search for similar objects "with a few clicks" (e.g. I'm looking for a ring similar to the one I'm wearing or that I found on the Auction House and with this option I just need to click on that ring and on the search button to get the results)
- (Based on the idea above) When we're going to sell an item, let us know the average price of similar items
- Make the "page" where you get the results of your search a "scroll-down-facebook-style page" where it loads items as you scroll down so you can have everything on the same page.
- Allow us to be able to "follow" the auction of an item without making an offer (something like e-bay)
- Let items with the property "reduced level requirement by X" to be displayed on the correct level range (e.g. If I auction a level60 item with a level reduction of 5, that item must be displayed if I search a level range between 50 and 55)
- Add a new type of AH where you can trade an item with another item (this last item will have stats, minimum stat value and item type - eg. sword, chest etc - chosen by the seller)
- NEW - Add the filter "average damage" even for mojos and wizard orbs
- NEW - Add a button to reset filters
- Add an hotkey to be able to swap between two "specs"
- Add "skill runes talent points" that allows you to improve skill runes
- Make skill runes you use "gain experience" so they can improve over time.
- Add the possibility to specialize in some skills (e.g. you can activate 2 skill runes for your primary skill by paying a price or by dropping and reading a particular book - this could be done for mercenaries too, to make them a bit more powerful)
- Remove the "add socket" property and make it similar to how it worked in Diablo 2.
- Make the "add socket" property on items of level 50 or above to be always followed by another magic property (this is to avoid things like this http://oi45.tinypic.com/2agru4z.jpg )
- Make the "increases the range of globe pickup" property to affect abilites with area of effect.
- Add the "increases block amount by X" and/or "increases block amount by X%" property/ies
- (based on the idea above) Make the "increases block chance by X%" and "increases block amount by X" properties to affect a character with no shield equipped. This might be considered as a parry with weapons
- Make the "damage melee attackers by X" property to affect ranged attackers as well
- Make "Crowd Control" properties (such as immobilize the target, or fear the target etc) more frequent by increasing their % to trigger
- Remove "Crowd Control" properties and merge them into elemental damage: cold - slows, lightning - stuns, holy - fears, arcane - immobilize, fire - blinds, poison - DoT. The chances to trigger the secondary effects are increased depending on the level of the item and on the "roll" of the elemental damage (the higher is the elemental damage the higher will be the chances to trigger secondary effects)
- Add magic properties that give you a chance to do "something" (aka proc items - 20% chance on hit to increase your AS by X% for Y seconds, 20% chance on hit to cast Hydra, etc.)
- Add the "add 1 active ability slot" property. In this way you can have 7 active abilites. This property cannot stack and only legendary items can have it.
- Add the "add X passive ability slots" property. In this way you can have more than just 3 passive abilities. Every type of item can have it (from magic to legendary), but only legendary items can add 2 or more, up to a maximum of 3 (eg. magic and rare items can add up to 1 passive slot while legendary items can add 1, 2 or 3 slots)