Smaller electronics that rely on special plastic clasps or hinges, think ITX computer cases or something like a game console or laptop. 3D printers are nice because then you don’t have to play Ebay part roulette or hope someone manufactures the particular part that snapped in half.
It’s tools and behavior that will produce change. If we have the tools but fail to change consumer behavior, overall change will be negligible.
If I use a 3d printer to print all the replacement parts I would have had to order specially otherwise, I am benefitting from the tool. But if those facilities keep making the parts I used to order, and the market overall focuses on ordering rather than making, not much environmental benefit is made from me not having things shipped to my door.
That doesn’t condemn the tool or the market, but the problem is this technology can’t have greater impact unless it changes consumer behavior at a critical mass, rather than just hobbyists.