I have started an experimental version of Grafx2 that loses layers but gains animation capabilities. This means the program and user interface stay as simple as the layers version, instead of becoming exponentially more complex (and also that I could do it in one week instead of one year!).
The experiment starts here, but if you want to test, be sure to pick the latest attached version. It's a very early version, waiting for your comments. By the way, the default shortcuts for previous/next frame are set to Insert and Home.