When zoomed in all the way on the timeline, add faint vertical lines to show frame delineations.
When doing precision work at the frame level, I think it would be helpful to add vertical lines to show where the frame edges are on the timeline when you're zoomed in to the maximum level. That way you can see if the slate clap is actually on this frame or the next and you can more easily tell how many frames you need to trim. It would be like the pixel grid in Photoshop when you zoom super tight. It would also be a good visual indicator that you can stop zooming in, because you've reached the max magnification.