I have thought about this alot over the years and carried alot of "stuff" that never got used... ever. If you carry a spare water pump then chances are it would be a hose or something that would go. So I re-thunk my thinking ( is that a word..thunk?) You can't carry all the tools you need for one and to try and anticipate what may brake is impossible and the diagnostic tools needed on todays cars and trucks...well. I carry some wire 12,14 and 16ga,some cutters a multimeter (which is the only tool I have used more than once) channel locks and crescent wrench and of course screwdriver and fuse. Plan for the minor things you won't be able to fix the major ones,you can only hope that you have enought MacGiver in you to limp to where a tow truck can get you.
I have had my water heater act up because of blockage and had yellow flames coming out of the cover and was able to fix it by running a piece of wire through the intake. oOn another trip I couldn't get my refer to light and cleaned the orifice with a piece of wire. You have an injector go out what are you going to do?Try and replace it? Not me not in the middle of no where,I will limp to where I can call someone. The best thing you can have is knowledge of how everything works then to know when you can do something. That is my 2 cents on this post.