It would be great to have the following:

1.  In the top level menu to have a 'Favorites' category for one's favorite channels that one could join with one click.  Just like it is done in a browser for favorite web pages. 

2.  In the top level menu to have a 'View' category, again like in a browser, where one could have all the GUI configuration options collected in one convenient location. From here one could control all panes, panels, tool bars, buttons, and everything else that has to do with appearance of the screen.

3.  In GUI options, to have a quick way of switching from 'tree' server/channel layout to the 'tabs' one - again like in a browser (Firefox).  Such a switch would allow to close the tree window and save space on the screen if needed.

PS: All these options have been executed in exactly that manner in another irc client - HydraIRC and as someone who used it for over 6 years, I can attest that they are really convenient.)

4. Another nice GUI feature that would allow to save screen space:  in the button tool bar, during a mouse over to have text describing the button appear just below it in a small window rather than at the bottom of the screen in the IRC status bar.  This would make it reasonable to close the IRC bar and either have more lines for text in the main window or just have KVIrc in a smaller window on the screen.

5.  User profiles: once several user profiles are created, it would be nice to be able to set one of them as default and to be able to switch them easily.  This way could go online with a diff profile with a click of a button whenever needed.