Information on using Ash Trees for Bonsai is hard to find. Most species of Ash have compound leaves which are not ideal for Bonsai. Compound leaves would look best on large Bonsai trees. There is a western species of Ash trees that has one leaf on it. It is Fraxinus anomala and wild yamadori trees can have incredible deadwood on them.
Be sure to treat your Ash trees with a strong systemic insecticide to protect it from Emerald Ash Borers.
Winter Fraxinus by Nik Art Bonsai Blog New 01-02-2020
Photos courtesy of Nik Art Bonsai Blog. This is the same tree in all photos.