the thing is, its important to pack lightly (if venturing far on foot) doing this will lower the load, and your consumption of resources like water.
that being said, there are a few items that you absolutely need in a first aid kit. first and foremost, a water bottle. this is good for keeping dyhydration away, along with assisting treatment for heat exhaustion.
second, a bandana or some sort of sturdy nonessential cloth. though bandages may seem like a good idea in these cases, they are not good for holding things together. if someone in your party, or yourself, happen to dislocate a leg, or break an arm, a bandage will not be able to hold up the broken limb.
I also strongly recommend an instant cool pack, and instant warm pack. these will help for injuries. keep in mind that these can only be used once.
it is a good idea to pack ducttape. it is good for sealing large wounds, and can also be used to pull out bee stingers and cactus prickles.
a phone. you NEED a phone, of course optional though. if a situation is bad, it is advised to call for some help from a medical pro. these fixes are mostly temporary, and can only last for so long.
dryer lint (or belly button lint) this is a very good firestarter, and fires are important to treat hypothermia.
a multitool knife. needless to say, these are very usefull to tear off clothing if a situation gets so severe that it is nessecary, like in the case of burns.