Don, your composition idea is really good, and with more of your tweaks, will make an excellent image...IMO.
Having said that, a few ideas that I had for you to consider: (Keep in mind, these are worth the price paid...) Oz
My suggestions, FWIW...
1. Crop just a little more of the trees on either side, which could crop more of the sky at the top, if you leave the crop size the same.
2. Unless they are important, clone out the two "Stobs" sticking down from the top of the arch framed tree limbs.
3. Use Nik Silver Efex control point (or other control point) to de-contrast and lighten the boulders and the trash can (area), so that they go away a little.
4. Use control point blur inside the tree frame to give the "Time Portal" look. Analog Efex Pro Zoom and Rotate Blur is an interesting one, and there are others. Just select the area to be edited in PS and use the blur tool in that area where you want it. Zoom and Rotate will let you keep the center (as big as you want) focused, and blur the rest of the selection area as much or as little as you would like. That way you would have a clear center, with the rest of the framed area blurred some, to send your eyes straight on through the portal. (Way too much coffee this AM?)
5. My choice would be black and white, instead of the green tone, for inside the tree "Frame"... or, perhaps sepia, or some "Old looking" duo tone... I think color for the tree frame out is great, perhaps a little more saturation...
6. If you do not have a dropbox account, I suggest to get one (It's free), and upload original full resolution file, and let the members download the file, and have a shot at helping you...or not...that does open it up to crazies like me! LOL