I was thinking that large icons that take up a lot of space in the viewports, you could also make an option for having toolbars at the top or bottom of each viewport and put thin text only buttons on these toolbars. One button you could add is having a minimize/maximize button on each viewport so you dont have to go down to the bottom of the screen to access those buttons. There should be an option to have no viewport icons or toolbars for those like myself who are satified just using RMB, MMB and scrollwheel to handle all view manipulations.