Sorry did not read most of this, so forgive me if this is said before.....
I would love to see the existing characters, but have them branch off a bit into the other classes. For example, a Paladin and a Necromancer, could end up with a Fallen Paladin. (I mentioned all this in another area but I'm too lazy to look for it)
Basically instead of summoning a Golem, you would summon a spirit horse in three different lvls. (this could also be rode upon and will add def to yourself) Have a lvl 5 horse, but it will ride odd and swurve off a direct route, lvl 20 a steed fast strong hit points def ect, and lvl 30 a nightMare which atcually does some minor attacks with its hooves and better stats then the steed. Auras will be more like curses that will effect enemies, or enhance it's own gear and abilities, like a poison summoning sword that uses your life to increase damage. Earth element summoning hammer that causes knockback/stun (each lvl adds a bit more poison damage for better durations), but lowers your defense a bit. Cold mace that can cause shatter/orbs on striking and as well as frozen, and increased length of slow, this could be taken away from your life with each strike like sacrifice does.
Another idea I had was combining a Sorc and a Druid, to become a Mage which is a male verson of the Sorc but relies on elemental allies. Earth, Poison, and Wind. Will have skills such as Life, and mana regeneration + to specific elemental damage, and lastly complete immunitie to 1 type of element (in stages of course)
The Amazon class could be split into 2 different characters Female (Javalin) Male (Bow/Arrow) This could be known as the Amazon, and Ranger classes. With existing skills, but just revamped a little with some new skills added to fill in for the lack of the other class.
The Assassin, could possibly have a split as well. Might sound a bit corny but I like the idea just need to work on a back story of where a Shadow master becomes its own person using traps, and the Assassin has to rely on martial arts.
Something I would love to see would be a Whip class and a character to go along with it. It could have taming abilities where the character could possibly control monsters for a short period of time to long legths almost like the Paladin skill conversion. The ability to raise the contolled dead like reanimate would be nice if they could have add on powers from skills like + to damage, life, def ect.
To solve the debate on having non human characters you could always have it when a character defeats hell baal or whoever is the last boss they can choose to have the same character only as a demon. Example would be a Minitor for a barb, Vampire for a sorc, ect.
I also believe they should upgrade the merc situation as well, to add a summoner of some sort, a cleric for those melee characters, and a tanker for the casters. If you think about it what is a assassin, but a hired killer. Imagine a low lvl trapper for a merc, or kicksin for that matter. Maybe another solution for the non human character idea, is have these monsters from D2 captured and sold as mercs. Those frozen bigfoot things would make a awsome tank, to where a shawman could revive the monsters your character has killed to aid in your fight.
Just because gold has become such a useless thing in D2, you could have a more indepth personalization to your gear. Change the colour to match everthing else you were, add spikes to armor, pay for upgrading of all gear. Name non unique, set, or magical items to say any name you want (within limits of course). Maybe even a NPC follow you to hold extra gear, or help you find runes and MF.
A bit of a side track sorry but I believe their should be a seperate area for dueling, like in a collusem or arena where its an automatic hostile on every character within the game room, and leave questing to non hostile only to prevent PKing (this is a must in any HC game). Within this arena you could possibly get a prize (such as a unique item only available to duelers, that cant be traded like anni, gheeds) for winning duels. You could have a preset area for team duels, merc and player duels, only one type of class duels ect.
Acts should have a non set path that a character can choose which quest to better fit his/her character. Malace for example who actually goes and gets that in norm nm and hell? I have yet to touch that quest outside of resetting of ladders when no one has any really good dear.
Lastly change the enviornment and battles in the different difficulties. Make normalin one area of the world and nightmare and hell in others. Maybe have a necromancer town or a magical town where their are alot of sorc, mage and warlocks, jungle like area where amazons are from. Also have the enviornment more interactive. Have a swamp that slows down characters, blizardous mountain top that decreases defence a bit, ect.
These are just some ideas I had hope this brings something to thought for the rest of you wondering what could be done. Hope you enjoy.