I am so, so sorry. You have done so much already, and there are some wonderful ideas here.
I would also say special recipes or family recipes. They may not seem important now, but will be when they are older. And lots of pictures, my mom did not like having her pic taken and I wish we had more with her. She did have a lot taken with the kids in her last few years though.
My mom passed away when I was older, but I often wish I knew more about when I was little (stuff that a mom would remember!) - like, "oh you used to do the same thing as your DD!" type things.
Good luck to you and your family. I cannot possibly begin to imagine. P&PT to you all.
"Every mother needs a wife." - Amy Poehler, Yes Please