I've always wondered what the point of carrying a tool set in a modern car is. How much can you really fix on the side of the road?
True, but I prefer to have tools on me nonetheless. they get used more on other people's cars or at parents house than on my own vehicle.
My recommendation is think of the likely scenarios you'll encounter and can take care of on the road and build our from that:
1. Flat tire = tire repair kit, 12v compressor. If the wheel has to be changed, a 1/2" breaker bar with the deep socket for your vehicle. Upgrade to a multi-size lug nut socket set to cover all your vehicles, or give yourself the ability to stop and help someone else. If you're in a truck or SUV, consider upgrading your jack. You might want some gloves and if you have to wear dress clothes to work maybe some knee pads or a mat to protect your clothes
2. Blown radiator hose = Duct tape and hose clamps will usually get you to an auto parts store. You'll need water, or something to carry water in. They make fold-able "buckets" for camping that you could keep with you to get water to your vehicle, or a jug or two of water if you're in a truck/suv that could also be used as emergency drinking water. You may need screw driver and pliers when you get to a parts store to replace the damaged hose.
3. Stuck thermostat = Socket or wrench that fits the housing bolts and a small pry tool. You then have two options: (a) carry a spare thermostat and gasket or (b) carry some RTV or thermostat gasket to remove the thermostat, bolt the housing back on and go W/O a thermostat. Option (b) would be if your size constrained.
4. Dead battery = Jumper cables or booster pack. carry sockets needed to replace the battery.
5. Consider carrying a qt of oil, and put the bottle in a sealed plastic bag
6. carry sockets for the common fasteners on your vehicle
If you carry just the items you'll absolutely need, you can make a relatively small kit in each vehicle so you're never without the bare essentials. Then if you have more money you can spend and more room you can carry more tools to expand your capabilities.