This is what I'm doing with my Bresse. I'm using 4 of the best roosters with 10 hens each of another breed, one of which is Delaware. Not sure of the other 3 but I will work it out. White Minorca may be a good choice if I can get the pullets. The White Minorca is threatened also. White Marans could possible be another as even White Leghorn could supply a few genes to start out with. It's a trial and error project.Piggybacking off of that thought, rare breeds that have went through a genetic bottleneck often become unhealthily inbred and are in need of fresh genes via outcrossing; something many breed enthusiasts are loath to do. 200 years ago, outcrossing was more common when “breed” was considered a fluid concept.
So sometimes saving a breed means temporarily mixing it with another breed, then line breeding it back the breed definition.