I hadn't even thought about the images relating to the timeline, but I see how that might be valuable. All for it!

I'll just toss in the mockup here, since this seems to be the main thread on that particular topic.

A pixel overlay to rough out key poses would be super useful.

Nothing fancy, just the ability to lay down and erase a line on top of the UI. In that way you could draw a couple of roughs and then pose the skeleton to match them.

A color selector for those lines would be nice to have, but not mandatory.