If you think about that there is little advantages of a customizable tool bar, everything that could possibly be there is already there.
An idea however would be to have the possibility to add buttons linked to a script. That is I could add a button N on the toolbar, when I click the button N it executes script N, script N is "tnick MyFavoriteNick".
Please add an option to remove/disable the Tile and Cascade buttons, I don't use Mizimized windows, I like them sizeable, I size them and dispose them very strategiclly, and I hate hitting Tile/Cascade by accident. For those in the same situation as me, a solution is: from any window, from the system menu, select Position > Remember All now! The next time you hit Tile/Cascade by accident go do Position > Reset All to restore them to the size/position you saved them to.