I definitely WOULD NOT want 3 bars on screen at all times or 30 buttons at the bottom as some people have requested.
Moddable to preference would be nice, yes, but how feasible would that be? It can be pretty daunting on UI design, it's not that easy.

Personally I wouldn't mind having an option to display 3 bars at all times (or just flip them up pressing a button onscreen or hotkey). I just know I wouldn't use it.
The biggest problem so far STILL seems to simply that the next and previous buttons are too small. If that's changed, and for those really wanting it there are configurable hotkeys all issues would instantly vanish right?
Minor tweaks... no super-overhaul needed.

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- Inventory is a mess despite the filters (that keep reseting... ridiculous)

What keeps resetting? Please tell?
Once you pick a sort, it stays that way. However, new items do get added at the end (not at the location it should be for your sort). Is that what you mean, you want it automatically updated each item?
Also if you make a gap (sell items, use item etc.) you have to note the UI doesn't automatically update, and new items will fill those gaps.
I have never seen an inventory system with sort that automatically ordered each and every pickup or other mutation though, if there is any show me which.
If I misinterpretated what the problem was, let me know...
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- Inventory and toolbar are not linked, I'm so bored to have to F1/F2/f3/f4 or click on tiny arrows to make everthing correspond

I do agree with this one.
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- No "common" chest for storing loot and access it when crafting

Typical MMO-mechanic added to please lazy players of the new generation. Welcome to old-school. I definitely don't want any mysteriously shared container that is magically used in the middle of a warzone when crafting even if miles away as modern RPG's sometimes have.