Let's fight about it
Diablo 3 has had quite the comeback since its shaky launch. Blizzard slowly but surely picked up the pieces, responding to fan criticism to create the Diablo 3 that everyone wanted. The Auction House? Dead. The loot? Fixed. The difficulty? Balanced. The Diablo available now is a much more complete experience, and one that any fan of the originals would be happy to spend a few hundred hours playing.
The expansion looks to add even more, including a new class in The Crusader. Will that be the last expansion? Will the Crusader be the final character added to the game? It's entirely possible, but that's not going to stop us from talking about other classes we'd like. Hell, with the Auction House going down, Blizzard might be looking for new ways to monazite the game, and we wouldn't be against spending $5-$10 on a new class or two. Here are some we'd gladly pay for.
If Blizzard adds the Necromancer into the game, we think he should be more focused than he was before. The class was spread too thin in D2, with an assortment of complicated spells, golems, debuffs, and summons. Strip all of that unnecessary fluff away (or make it less of a focus) and bring it back to the core necromancy experience: raising armies of the dead. Skeletons, zombies, corpses of fallen foes--the Necromancer should be surrounded by shambling undead at all times, protecting him from enemies while dealing damage back. He should still have some spells and debuffs (Corpse Explosion and Bone Spirit are a must), but they should totally be secondary to his main role as *lightning crash* LORD OF THE UNDEAD HORDE *lightning crash*.
Tear away his elemental spells (seriously, throw em right away) and cut off the summoning completely the druid should be all about changing into animals. He could turn into a werebear to get a huge armor and vitality boost, a rampaging wererhino that charges and stomps on enemies, or into a werewolf in order to gain some increased attack speed and lifeleech. Each form could have its own abilities, specialized to the animal he shapeshifts into, making the Druid more flexible than the other classes in the game.
In a way, he could almost work like the Engineer in Team Fortress 2--creating buildings that help his team, while placing and repairing devastating, upgradable turrets in key locations. These would function like hyper-powerful versions of the Demon Hunters Sentry, with the added bonus of being able to place several different types at the same time. Itd be tricky, but if done right, the Artificer could provide an awesome deploy-and-protect D3 experience.
To differentiate itself from the Monk as much as possible, a Diablo III-era Paladin could be portrayed as an overly fanatical religious warrior, similar to the Scarlet Crusade in World of Warcraft--or, better yet, the same Order that the Templars Kormac and Jondar hail from. Though they may slay demons and protect the innocent as a result, who knows how long itll be until their devout faith drives them over the edge?
Porting over the Amazon wholesale to Diablo 3 wouldnt make sense, but perhaps a new class, like "Dragoon" or "Lancer" could take its place. With javelin abilities that emphasize poison and lightning, as well as D2's awesome Dodge, Avoid, and Evade passives that let the Amazon duck under and around incoming projectiles, these javelin-flinging jockeys could introduce quite the varied playstyle.
Things would really get interesting when it came to the Outcast Devils abilities. Without limitations on human form or mundane magic, wed want the Outcast to transform his or her body at will, shifting an arm into a barbed, long-range tentacle or temporarily morphing into a nightmarish beast akin to the Barbarians Wrath of the Berserker. Add in a host of hellfire-based spells, and youd have an awesome melee-magic hybrid fighter.