For my workflow (and maybe for many others as well) it would be really handy to have a button/shortcut to toggle between the current tool and the previous used tool, instead of having to scroll through the list of tools to select the previous tool. I would imagine this be a button sitting in the ‘shortcuts UI panel’.
I would use this a lot for example with the mask tool i.c.w. A tool like the clay, move or inflate tool. I would often adjust my mask and move or inflate a bit, and go back to the mask for another adjustment, and so on…