Ideas are cheap, but here goes :)
In my workflow I usually want to end up my work with applying a global effect on the whole timeline (like sharpen).
The easy route would be putting this into the rendering dialog. Something that would require substantial UI design would be having clip groups a first class citizen. Being able to apply effects and transitions to groups of clips just like you can on its children. But that would require to figure out how to "enter" the group and work on the clips inside.
Right now you can only paste an effect on all clips, but that leaves out transitions and makes it difficult to toggle an effect off to speed things up while working.