It would be a very handy feature to have a checklist box of undos to choose from.
A few big things for me would be to leave the option to turn off undo from anything plugin related, with the EXCEPTION of being able to undo if you accidentally delete or move a plugin. THAT'S a good one. Other than that, most plugins have undo/redo buttons built in. This would help mixing workflow so you don't have to worry about getting into a situation where you're undoing your mix to simply get back to a different panning position or fader level.
I'm sure others could find other practical uses for undo settings as well. You could have a dialog box to check or uncheck things in the preferences for example. I believe Cubase has extensive options like this if I'm not mistaken? Could be wrong.. if I am, plus one for Studio One, am I right?? If I'm right, well one step closer to being the daw that does it all