Love your plan!
For an art quilt, it is my thought that the sky is the limit on what you want to combine to create your project. Go for it!
For the leaves, you might be able to find some in the silk flowers at Hobby Lobby or Michael's that would already have the color shading that you could use and satin stitch around, then determine your remaining scale from that.
Maybe use pearl beads, velvet and gold lame' for the crown, fake fur for the buffalo...
Oh, this is going to be gorgeous!