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.

All true! I don't know why at least one of those possibilities didn't work for me before, and I can't even reproduce the message I was getting...   What about having 3 columns showing all possible slots always, with the non-functional for the given selection being greyed out? After all, that's what happens with the other actions (the construction panel). This would also show the player all that is possible. The possibility I like less is not showing buttons for actions that can be performed, it's not about knowing the hotkeys, it's about clearly showing new players what can be done. I think it's good to have the option to hide certain things.   Well it had mostly vanilla in mind, and the row ordering criteria I defined was quite generic (mostly economic, civic, military, and special), but yes, those new classes would be needed for it to work.