Welcome to WC!
The way you used the markers in your sketch gives this piece a very graphic and illustrator feel. If you will be doing the final maintaining some of this technique, I'd recommend using acrylics. Then if you will be doing a more painterly approach, the oil pastels would bring more of a soft-edged depth the piece (not that you can't also achieve the same using acrylics.)
Cheryl's observation about the heights being equal was good. In addition, the fact that the building has a peak just like the mountain to the left gives a pointed feel to the piece. Perhaps emulating the mountains on the right in your mountain on the left as well as dropping it down a bit for depth.
The tree has a very nice shape.
On the fencing, if you will be leaving both sides of the fence in, just adjust the shapes so they work with the painting, directing your eye more up and to the house.
Please let us see how you continue with this one.
L. Diane Johnson
Plein Air Workshops