I guess I'm really talking about those progression mechanics that I didn't really like in TOW : the fact that weapons had levels and the poor perk selection.

I hope that we have a greater selection of unique weapons with unique effects or mechanics, and no levels.

As for the perks, I guess there is an easy fix : excise the perks that are automatically tied to skill progression and fold them back into the perk list.

I found it a little frustrating in TOW that high-level weapons were locked behind a lot of grinding. In Diablo 4, I'd love to see a system where weapon rarity determines power, or where we can craft unique modifications to personalise our weapons.

I agree, I think weapon shouldn't be levelled - *everything* must come down to your build.  

Also wider selection of weapons and more unique weapons supporting more builds. 

The capability to mod your weapon is already good, I don't think it needs further improvement.