Glad you decided to leave it as a cake... the tree theme just didn't seem to go with Halloween. It's crazy and chaotic... just like halloween.
I agree you will like it better after quilting. Do finish it. You could do some really cute quilting on this... like an outline scribble in white to outline the edge of the cake and make it stand out from the background. I'd go around it three or four times without putting the stitch lines directly on top of each other, for that 'scribble' effect. Then you could quilt some scallops along some of the 'layers', like frosting on the cake. This could be adorable.