How can I use self made cream of celery as a replacement for mushrooms in green bean casserole?
After reading your original question, the revisions, and the comments, I have what may be an option for you. As noted in comments, you can use just about any vegetable you want. 

To keep the flavor profile the same, make the creamy sauce as you usually do and then strain it well through a colander to remove the mushrooms. (You can also do this with celery.) By doing this you will have the flavor of mushrooms, celery, or whatever vegetable you choose in your dish.

The other option would be to purée your sauce.

Then, if you want a vegetable (of your choice) to add for texture, sautée them in a very small amount of butter to your desired consistency and add to your casserole.

If you don't feel the need for adding another vegetable, simply don't add any.