I think it all just depends on how much you really want to plan for and what size of kit you want to build. First things first, medical should be at the top of the list. It may not be the first thing or something you won't ever use off road, but it only takes that one time for an accident to occur and there's some significant bleeding or fatal injury, chances are that help is 5 mins away if off road is mute. Invest in quality materials for a medkit, USA made ia recommended, avoid the Chinese stuff as moat of it fails when it's needed most. The kit also has to be adapted to your vehicle. Does your vehicle have a winch? The time of year or season also makes a difference, a blanket or some type of gear can make the job smoother or bearable such as mosquito spray or poncho while other gear can be life saving such as winter gear for cold climates/seasons such as gloves/hat or blankets.
To help give you an idea, here's what I did.
For my deuce, I have a 3 piece kit due to the amount of room I have. A tool box for recovery gear such as shackles, snatchblocks, rope dampers, straps, flares, glow sticks, E-tool, hatchet, and folding saw. The ammo can has water/food, medical gear, and flash light, lantern, and extra batteries. The 70L pack is filled up with straight up survival gear to allow me to get through a few days should I not be able to recover or have to stay with the truck until help arrives or leave the truck on foot. For most of us, our main tool for help is our phone, so make sure to have a way to keep it charged such as a battery bank or solar charger should be something considered.
My pickup has two tool boxes on the sides of the bed, so I use one that has a small pack that is a survival kit, but also have a snatch blocks, tow strap, three shackles, bottled water, couple MRE's, hatchet, flashlight, E-tool, and IFAK. Usually in the winter months, I lay a wool blanket down on the back seats so it's there but out of the way.
Also carry extra fuel which should be a no brainier, but many seem to fail to do so, especially city folks who end up getting stranded in a winter storm on the express way and end up freezing to death once their vehicle runs out of fuel.
Things will also differ depending where you live, but believe it or not, there are some locations that are remote enough, help can be out of reach due to the lack of reception and in some cases, help can be a day away. This can be extremely dangerous in desert or winter environments, so plan accordingly, have a plan, and make sure someone knows where you're going and when you plan to return.
