Sometimes it takes for me minutes to reach the needed point of "Undo" - my computer is 6 years old, and it's hard for him to handle with ArtRage. Because of "lags" I have to sit a long time, holding/pressing Ctrl-Z and waiting... Then, pressing Ctrl-Y because I missed a moment ^_^"
So my suggestion is to add a table for quick returning to some key moments in painting - Adding/Deleting layer, drawing with other instrument (for example, I sketched with Pencil, and when I switch to other instrument and make a stroke, a key Undo is added), maybe some other...
It would be like a History bar in Photoshop, but not with a hundreds of actions - only the most "splecial" be there.