What a great idea! Vintage - hard to find - needles, bobbins, feet; these come to mind quickly. What about a kit with instructions and products for cleaning/servicing machines for those brave enough to do that? (I haven't gotten to that category yet, but can't find the products you have mentioned here in my neck of the woods.) I am sure there are a lot of other things...
At this point I still am lucky enough to have an OSMG near enough to travel to, but I have no idea how long that will last. I am glad you are going to do this.