Here are a few rules of thumb.
There is no such thing as the perfect kit. Your kit should change based on your trip. How long you are out where you are going who is with you etc.
Never carry anything you Do not know how to use. aka if you see it in an ambulance or ER and your not a doc or a medic you most likely should not be carrying things like IV's suture kits and big gun drugs.
Carry multi use items. For an ex carry a leather man vs a knife, saw etc.
Carry things that you simply can not improvise like medication, tape, irrigation syrgines etc.
Ask your self this question if you could only have four items in your kit what would they be? This really helps you narrow things down and get your thoughts in order.
For me it would be
Tape
Ace wraps x2
Meds
Irrigation needle
Every one has a bit of a different answer I guy I work with teaching wilderness medcine says trip wire, a knife and oxy. But then again he is a former airforce PJ and wicked hardcore.
you can improvise ALMOST everything else with what is in your pack and truck or in their pack and truck.
Finally the best thing you can carry in your kit is training. If you have not already, you should think long and hard about becoming a Wilderness First Responder (WFR) It is the gold standard of back country care. The entire class is based on assessment, and treatment, and almost more importantly making decisions and judgement calls and improvising.
If you are on a budget with time and money you should at least have a WFA ( Wilderness First Aid) it is a two day course and costs a few hundred bucks.
it is a great entry level training but keep in mind it is more of an awareness siminiar for the weekend warrior.
Where am I coming from?
almost ten years as an EMT and at one time an ALS provider.
Years of experience as a wilderness EMT.
Instructor for NOLS Wilderness Medicine Institute
years of extensive backcountry experience.
One of our staff recently started a company and sponsors these courses based around and for overlanding and adventure travel
check it out.
http://advmedicaltraining.com Adam Timm is a solid guy with a strong skill set.
also check out NOLS.edu/wmi for a current offering of all sorts of course types and more info.
Education is the best part of the first aid kit
cheers