Being able to vertically lock sections of the timeline would be HUGE. Always afraid of accidentally messing up sections I've finished previously while i'm working further down the line. You can lock tracks horizontally which is very useful, but being able to lock clips vertically would be incredibly useful also and give us peace of mind on big projects
R S commented
A more sophisticated grouping tool combined with more sophisticated lock functions would be very useful (in lieu of, or as an expression of "vertical lock" asked for above).