I think you're onto something laotin!
Back towards the beginning of the game after launch I had a UI setup similar to this, except I like to have two main chat windows on the left and right side, with all other pertinent windows arranged in between them. I used to have my maintained spells window right above my hotkeys and right below my targets because I just plain needed it there front-and-center =\. As my UI evolved I pushed maintained aside, had to add a fourth row of hotkeys and more detailed (read: space hogs) target/player windows....now everything is just too cumbersome to fit the way hid like
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Anyhoo, I like your drawing, and the layout seems pretty good. That group window layout is exactly what I prefer and use atm. Attached is a picture of my grp window for reference. The buttons on the left are assist buttons for each grp member.
The only thing I'd really need to change up front is splitting that big chat window up into two windows. I like separating melee and spells (too much spam), and I like to see the damage I'm doing and resists I'm getting (I also play a warlock, so I'm not always stuck in healer mode). Also I play in 1600x1200, perhaps that gives me a little more room to work with than you.
The window you drew for the primary, secondary, activatable slots etc. is a great idea, and reminded me of the Vert hotkeys window from his EQ 1 UI.
Here's a shot of it. On the left of the picture you see the hotkeys plus another bank of buttons right next to them. Those were customizable "clicky" slots, and he even included a little program you could run to set which clickys you wanted, and it saved it to the file for you (no xml knowledge needed). It would be awesome if such a thing was possible in EQ 2, but as its own window, or perhaps part of the menu button. An example of what slots I'd want as a healer would be 2 activatable, primary, secondary, ring, earring and one bag slot that hold my switched out gear.
Any of this jive with you laotin?
Any modders gonna run with this UI idea?