I downloaded the latest alpha, and I had two suggestions.
1) If you want to make an animation that involves simple flipping of a part, i.e. making someone's head look back and forth, you end up with a tween that makes the head literally flip like a piece of paper.
In a frame-based timeline, what you'd do is have the frame just before make the head face one way and the frame just after, make it face other way. But it's a little more trouble with the seconds-based timeline that Spriter has.
My suggestion is - could you make it so that you can drag pre-flipped versions of your images from the file list into the screen, and have them be considered separate ID's from the non-flipped versions?
2) Spriter could be used for full-screen cutscenes, as well as character animation. If you want to use it that way, often you're also working with a background audio track that you want to sync with the action.
I know that you're planning to have sounds play during the animation preview - but could you make it so that sounds also play when you're scrubbing the timeline?
Thanks for making Spriter.