As I find myself ordering things again, find myself thinking of what emergency parts that would be good to be handy. I think I have a decent list of what to carry when I take longer rides, but there are still things that can break that are worth the investment (i.e. probably need sooner or later) and worth keeping on the shelf to avoid shipping time and expense for a single item. Last year after busting a fuel injector and finding a cheap replacement, I'm keeping one of those on my shelf. I usually keep old belts for emergencies and buy new as I near the end of a maintenance cycle, but I'm sure I could consider others.
What I have now...purchased or gifted:
Belts, exhaust gasket, fuses, on/off switch, spare bulbs, various screws and other fasteners that are likely to disappear from various causes.
Anything else any of you keep handy?