A few key things that aren't fully covered in the thread linked above. For any sort of first aid or medical, make sure that you a) know how to use every single thing you pack into the kit, and b) you build the kit yourself. It's fine to buy a kit, but take it apart completely and reassemble it keeping the first point in mind. Make sure you know where everything is in the kit, and you can find it in a stressful moment while adrenaline is pounding. Source: I'm a trained wilderness EMT.
As for other emergency equipment, I bucket it into a few categories: Mechanical, Electrical, Safety, and Comfort.
Mechanical will have the basics you need to get the truck moving again in a minor emergency. Think things like a good socket set, screwdrivers, etc.
Electrical includes a box of spare fuses and a pocket multimeter. Make sure you also have a battery jump pack - battery technology has gotten good enough that jumper cables are pretty unnecessary these days. Just make sure you keep it topped off once a month.
Safety has all the stuff that you need to safely use other parts of your emergency kit. I keep a headlamp, emergency flares, emergency LED beacons, reflective vest, good leather gloves, and several pairs of nitrile gloves.
Comfort is the stuff that will keep you from losing your mind while you wait for AAA or whatever else to come rescue you when you're out of your depth. Wool blanket, instant-hot packets/hand warmers, deck of cards, long-term stable food like protein bars. Boredom can kill just as easily as cold or heat, so make sure there's something to keep you entertained if you get stranded for hours. I also keep a book in my truck, but cards are a safe bet.
