A lot of good advice has been posted here. I do have a few things to add.
Hit the U-Pull-It for a humongous handful of radiator hose clamps in all sizes. Get lots of them. You can link them together if necessary and do amazing emergency repairs with them.
Get a six-inch length of old bicycle tube for making hose patches & rubber bands.
Get a big handful of metric & standard nuts and bolts, too. Keep them separate, with the nuts & washers already threaded on the bolts. Ditto fuses, light bulbs, spare wire, fuel filters, etc. All the small stuff you may need desperately on the side of the road.
Snag an complete dome light, and make your own 12V trouble light.
Carry both metric & standard sockets & wrenches. You may need to be able to work on both genders. Get a universal joint, a magnetic pickup, and an inspection mirror, too. Also one of those spring-loaded grabbers.
You can cut the bottom off one quart oil bottle, the top off another, wash them out and telescope them together. The resulting container will hold all the small stuff, and provides both a funnel and a gas-catcher at need.
Cut the tops off two gallon-size anti-freeze bottles, and you have a primo tool box, plus almost two gallons of oil-catching or water-carrying capacity. Or gas-catching capacity if you are salvaging gas from abandoned vehicles.
You can also use that gag for waterproof containers. Just duct-tape the seam. Great for keeping clothing clean in the trunk of a car.
Pack gloves, small towels, and a tube of Goop for cleanup.
Carry a 12V soldering iron, 5% silver solder, and rosin flux, plus a bunch of electrical connectors of all types.
Carry also a honkin' big coil of baling wire and buy a Stronghold Haywire Klamper. (See the video on YouTube.)
Pack a hacksaw & spare blades, and a cold chisel & hammer.
Carry at least one roll of Rescue Tape. (See the video on YouTube.) You need a roll for your first aid kit, too. For sure.
And duct tape & electrical tape, of course.
Carry 12" lengths of small diameter tubing so you can repair vacuum leaks, etc, plus straight connectors. A yard of heater hose can come in mighty handy, too.
Gasket sealer & thread-locker can also be useful, and don't take up much space.