Spare Thermostat and a Radiator cap might prove handy to go with xtra hoses, belts and coolant.
Consider checking to see if your vehicle has any special needs for fill & venting the cooling system. My pickup does .. but no car I ever had before did ...might be handy to know.
I don't think thermostats fail as often as radiator caps might.
And I've had about every water cooled car have a radiator failure. Most vehicles now have plastic manifolds on their radiators ...the one on my truck leaked before 50K miles. My other failure experiences were with metal radiators and occurred around the 100K miles mark. I'm thinking now maybe some kind of hi temp epoxy repair system might be helpful for just such a failure.
Might want to add a set of Torx bits ... I discovered I needed mine when I changed out my throttle position sensor last week and saw more of those type screws than I'd thought there would be.
All my cars carry a small air compressor and Flat Fix. Some compressors are wimpier than others ...mine cost $50 at Costco and is the strongest I found so far. There are more expensive ones out there .. but $50/car was spendy enough for my wallet.
I love my Cold Steel Shovel ...but a folding one might take up less space and should work adequately enough.
Hadn't thought of adding an oil drain plug and washer ...that could be tough to do without if it got lost.
I don't EDC or stash in my kits any less than a week's worth of personal Meds....and a month is my preferred minimum for my kits.
Love the compartmentalized examples that were posted ....some day my stuff will be that organized too ...