I have a different twist to this. I would like to see a mode where all the tool paths but the current tool's path are ghosted. A user settable ghost percent so you can fade it a little or all the way to black. And I would like it to work as you are scrolling through the code after having just clicked a "Refresh Plot".
My preference would be for it to decide what is a new tool based on seeing an "T" command. And that it would not care even if the current tool is the same as the previous tool number.