I like the onion skin bar 
Concerning frames preview, I’m not sure.
On small animation it could be Ok, on large animation it will be resource killer; if implement, this kind of functionality should be activable/deactivable.
Also, you might need a very large screen to be able to use this (a 4K, because on a FullHD I don’t think there’s enough space to display things properly)
I understand the “Hold last frame” and “Stop” (except that “Hold last frame” should be default instead of stop for retro-compatibility with existing animation), but I don"t understand how “Loop” works.
I don’t understand this:
What colored bullets means?
What is the slider?
Grum999