It seems that the @nifa has already done some work.  

Exactly. For a game overlay, they are the worse thing possible.

The art on the walls should be updated for the following Roman factions.

And sidebars or overflow arrows? Which could be hidden but work by scrolling. But I have the feeling these are avoided, even when they could help reduce space used. 

If you are referring the @Thalatta's last mock, then the dead space would always be less then current ModernGUI layout (10 slots vs current 12) but it's a bit closer to screen center which might look worse as said above. Your other points I acknowledged them in the last comment, so agreed but you always have to make compromises, goal is to make the best ones.     We're back to first exchange : too much clutter, too much space used. The second option is just slightly better enough to make it interesting.