Repairs in the field.. not very practical and not many people can do them. Most people carry maybe some spare belts and a very specific part they know it is a weak point on the vehicle, and even that only on very long trips. That you can research on specific forums for your particular vehicle. For example, X model of Ford has a weak alternator, so you carry one as a spare and learn how change it - only that one. On Land cruisers bearings fail - so many people carry one as a spare. For another vehicle might be an intercooler hose that is prone to failure and so on.. But not more than that. Also.. tools - good to have some but to get too many takes a lot of space and they are heavy.
Second thing - unless you drive like an ape most mechanical failures don't happen overnight, you usually get some kind of warning , weird noises mostly, a light in the dash . Off road, most of the time I crack the window at least just to hear better. The moment you notice them - you try to determine what kind of issue is and head back immediately if you are not sure what it is and if you can make it. Get some duct tape and leak stop solutions.
What will stop you dead in the tracks are electrical failures - so get familiar with fuses, relays, where are the fuse boxes on your vehicle and how to test them, get some wires, a crimp tool , they are light and take almost no space what so ever. How to check fuses and relays you can find plenty of youtube videos.
I drive a Land rover, not the most reliable thing ever built and I just have an OBII diagnostic tool and electrical stuff, duct tape, some spare fluids, leak stop and that is about it... If I would go very remote and for very long time ( like months in a row, I would get maybe the suspension air compressor as a spare, and that is it.. )