No rules! It sounds like you have a good idea of how you'd like it to look, you just need to decide on a technique. Space between quilting is limited by the batting. Some batting will allow you to space stitching further apart. Do you think some of the big stitch hand quilting would look good? I don't think I could FMQ something that large on my domestic machine. If you're going to hand quilt, you might pick a batting that is recommended as easy to hand quilt.
A wonderfully complete and concise answer to your question. I would only add, if you're thinking of doing 'branch shapes', look into some of the various color choices you have for hand quilting thread as well. Determine if you want your branches to be noticed or not (I would think you would) and how 'earthy' looking you want to go. I love using various colored threads - even in a single quilt - to highlight or lowlight my quilting.