I used to have an empty 40 gallons tank too and I finally put something in it this weekend and it works perfectly

Low maintenance natural setup and my cresties are happy (just had to wait for them to be full grown before moving in)



There are many things you can house in such a setup, rats would just make it such a hassle on cleaning day that why I don't think it's a good idea, cleaning a big tank weekly to remove the smell of urine would not be fun.