NetDep - The biggest challenge, and I've spent hours with my girlfriend planning what to do in medical emergencies (as we're often 5+ hours from a properly-equipped hospital), is the balance between urgency and equipment size/weight. Yes, you can build an EMT-style "what's up" bag, but that's just too much crap to carry with you. People say, "Well, also, be careful to never travel alone." Frankly, this might work for aid workers who are in the region for a week or two, but it's total bull$*)# advice to give people who are in country for 15, 18, or 30 months. Often, when I'm on fieldwork, I'm alone (and often she travels alone). That's just the reality of it. And often I go somewhere and have to park the vehicle and walk, hike, or ride (usually on the back of a dirtbike) from the RV point to the meeting itself. So what to do?
Our solution is to have two sizes of first aid kits and to communicate where the vehicle is to at least one other person whenever we leave it. The small first aid kit is kept in the passenger door panel of the FJC. It includes light wound dressings for two small sites, two epipens, a modern (velcro-and-plastic-pencil style) tourniquet, paper/pencil, ten tabs of Vicodin, two small vials of stopbleed pellets, a small section of space blanket (because the stopbleed stuff gets crazy hot when used), etc. - and a map of hospitals and airstrips. It is a soft-sided rectangluar bag with a handle (an old London St. John's Ambulance bag, to be specific) and has lots of pouches. We've completely repacked it. It goes everywhere - if you leave the vehicle, it goes in your pack. When you pack your pack in the morning, you leave enough room for the small vehicle medkit.
I'd recommend printing (on A4 paper) maps of hospitals and airstrips in the area you're driving to, if it's a destination drive rather than part of a larger expedition - I have added several sites (including clearing sites usable for rotary aircraft) in northern Uganda to my Garmin map, and I'm happy to share this process (though it's navigation and I guess off-topic here?).
The larger kit (1520 Pelican) is marked with a large white cross on a black case so it's obvious to other people if we need to be treated by someone else with access to the vehicle. The load area of the FJC is always loaded so this is the last thing in and it is always loaded with the cross facing rearward. The case itself is packed with a HIV-endemic area in mind, with gloves on top as a reminder to anyone who opens the case. It includes pressure dressings, rigid (packed) gauze to pack wounds or stabilize impaled objects, and the other "heavy duty" stuff you should have on hand, but aren't going to carry on the hike from the vehicle to the site. It's prioritized around things we can resupply - for instance, we began with airtight dressing tape (basically a plastic sterile very wide tape) but plastic wrap also works as a replacement. Carrying flat scalpel kits (gamma/ozone treated sterile, blister packaged) rather than opening your suture kit every time you need a scalpel is smart. Carrying good tweezers (Rubis in Switzerland makes amazing products) rather than crappy ones, given that this is something you are likely to use, is a good idea. Carrying enough sterilizing agent (alcohol or alternatives) rather than tearing open swab or pad packages is smart. Injectables are an area of controversy, but we carry the Vietnam-era "vial box" rather than the modern kits, just because they are more resistant to damage (albeit heavier) - local anesthetic being probably the most versatile and useful injectable you could carry, if you only carried one.
I would say medical bag weight/space versus probability of use is one of the toughest calculations to make when designing a vehicle, and I really appreciate how much thought my girlfriend has put into this area, and how much work we've done together on figuring out various scenarios. Talk with your co-driver, navigator, partner, etc. Discuss what you'd do in certain situations and what kinds of risks are acceptable to you. Have a policy in place for others (when you encounter others who would benefit from your help or supplies) and stick to it when you encounter people who are injured (you will) no matter how tempting it is to intervene or how pathetic the person's (or the person's children's) requests for aid may be. Remember, Objective 1 is to get from Point A to Point B, not to hand out painkillers on the roadside.
If something is unlikely to be used, don't dismiss it. For example, a round Heimlich valve (flutter valve) is basically a flat sticker when in its package, weighs nearly nothing, and could save your life for valuable time in event of pneumothorax. So, in my opinion, it goes in the bag. Maybe someone misses you with a knife when a bar fight breaks out, maybe a large fuel or natural gas tank explodes near you (this has happened to us), or something else happens that generates matter with enough mass and energy to puncture a lung if you're unlucky. Don't put anything in the bag that you wouldn't feel comfortable using - that discomfort will be MAGNIFIED by a situation in the field.
Finally, be realistic and consider that you may be the casualty. Do not carry unmarked drugs (for borders and checkpoints, if for no other reason) and think in terms of someone local, with little education, treating you. Make it obvious what things are if someone else needs to use the bag. If necessary, draw a picture on a piece of white duct tape. Mark anything containing sharps with a picture of a needle. Include a card printed with the emergency numbers for the embassies of the countries to which you hold citizenship - better, include a direct consular number that you know a person will answer 24 hours per day, rather than something that gets forwarded to London (or Washington or wherever). Put a spare key to your vehicle in the first aid kit on a bright red keychain, so someone who dumps it out will be able to find the key and drive you somewhere if you're unconscious (we have a cheap keychain photo frame, like people use for pictures of their dogs/kids, on this key with a picture of the truck so they know which vehicle is ours). Though some dealers don't know how to do the programming, even the newest Toyota vehicles can have four duplicate keys plus the original key (we have five keys for our 2012 Tacoma).