The more layers you have, the more dimension and depth your piece will have. My Janome FM725, needle felting machine is what I use to create not only depth and dimension but also texture.
This particular machine has no bobbin and no thread. It has a five needle unit and a single needle unit. Each needle has barbs at the tip which is used for splitting and locking the fibers to the base fabric. In my needle felted pieces I use felt as the base fabric, then pile on layers of silk rovings and any of the following: wool rovings, carrier silk rods, degummed throwster silk, silk cocoons, scraps of fabric and sparkle threads. The company I get my silks from is Treenway Silks in Salt Spring Island, BC, Canada. I also use my hand-dyed cheese cloth.
I will either outline if I have an idea in mind or I will just lay the rovings down and start needle felting. I will keep laying down rovings, building up the layers, color and texture as I go along.
When I think am done, I will pin it to my design wall and live with it for a few days. If I am not satisfied, then it's back to the machine to add perhaps more fiber, cheese cloth, thread etc. If I like what I see I then sandwich the piece (batting and backing) and free motion quilt to add further dimension and another layer.
Now comes my favorite part. It's time to embellish with beads, hand embroidery and found objects which add yet another layer. I do enjoy hand embroidery as I find it very contemplative.