One way to freshen up the game's combat, and item/weapon progression, would be to give each weapon a 'weapon skill' that could be used in combat.
There wouldn't need to be a unique skill for every weapon - that would be practically impossible to implement, of course. But even a small set of skills could add so much to the game. Don't believe me? Let e tell you some of the possibilities:
One idea that's occurred to me as I'm writing this is to have an 'Axe Throw' skill on some axe-type weapons. This would throw the axe at the enemy, and then arc back to the player, dealing damage to any enemies in its path. This would allow skilled players to master the skill, using positioning and movement to maximise the damage that they do. There's plenty of other possibilities, that could suit different tactics in battle. For those who like to act as a tank, you could have a 'Draining Blow' skill that does unremarkable damage, but heals the player for damage fone. For those who prefer to kite their enemies, you could have a 'Dazzling Light' skill that blinds the enemy, reducing their chance to hit.
The game's combat gameplay could benefit a lot from this. Creative new combinations would be opened up, as players learn how to combine their weapon skills with the class skills to good effect. Players would have a more varied experience of combat, being able to try out new weapon skills to add variety to their usual skill rotations. And this would give players some more freedom in creating their builds - as it stands, two players of the same class will play very similarly, but the addition of weapon skills would really allow players to express themselves more.
When it comes to the item/weapon progression, this would add so much. As it stands, weapons of the same level are pretty much identical - sure, there's some stat differences, but these are fairly minor and aren't particularly noticeable in combat. The addition of weapon skills would make the choice of which level-appropriate weapon to use much more interesting, as you have to consider which weapon skill would be the most fun, or would synergise best with your build and your friends' builds. This would also allow for the really special weapons - such as the Doom Weapons - to stand out, as they could have unique and powerful weapon skills.
An alternative would be to add trinkets to the game, that have their own equip slot, and give the player access to a 'trinket skill'.
Armour Stat Variance
Armours of the same level tend to be very similar in terms of performance. This means it's simply a matter of grabbing the highest level of armour available to you, and running with that until you level up. But this doesn't have to be the case.
With armour, you could have armours focus on different stats. One armour set could focus on getting the Armor stat as high as possible, allowing you to tank lots of damage. Another set might focus on Evasion, letting you avoid some damage entirely. Another armour set could do less to protect you, but give more of a bonus to your Attack, Crit, and Haste stats. This would create trade-offs when choosing which armour set to use, and allowing players to express themselves. One of the game's greatest strengths is the fantastic visual customisation available to players - so why no go a step further, and allow characters to have individuality in how they fight, allowing my loincloth barbarian to really be a berserker, while my friend's heavily-armoured paladin can be a true tank?
Players, of course, tend to use a lot of cosmetic armours, rather than showing what they're really using. But this would still be a cool change that would give players more freedom and choice, while giving us more incentive to explore the full range of armour content that is in the game.