an option to fade the last process applied to the image, be it a 
brush stroke, an effect/filter, would be great. similar to in photoshop, if you 
apply a filter or draw with a brush, then you can go "Edit > Fade (brush/filter 
etc)", and use an opacity slider to fade how transperent the effect is applied 
to the image, so in other words you dont have to clone a layer, apply an 
effect, then make the top layer 50% transperent so you can get a subtle blend 
of the before and after.