I agree with you about the previewing... It's probably a major improvement, but it would be awesome!
I posted a nice way of improving the animation editor about a year ago, but it went mostly unnoticed, so I thought I should remember everyone about it
It's not the same as you are requesting, but I think this method I proposed would save a lot of texture space and video resources and, in the long run, would improve the way you animate.
After I proposed this I joined a game development studio, and I got to experiment on both Playground (a nice game engine by playfirst, free to use at http://developer.playfirst.com) and the Nintendo animation software package, and they are very similar to this things I tried to reproduce on Construct.
Have a go, the .CAP is old but it will run on the newer versions.
BTW, TIMELINE IS GREAT FOR ANIMATIONS!