I can't wait to see what you come up with, Julie! FWIW, none of my winter jackets have elastic at the hem. I think if they did, it would be annoying because the jacket would tend to ride up, particularly against the slippery Lycra of my tights. I have several jerseys with elastic hems with this annoying trait. My jerseys without elastic perform much better.

PS, I don't have a serger, but my sewing machine is from 1954. It works beautifully.