The likely treatable injurys are abrasions, a broken wrist or dislocated or broken shoulder/collar bone. Some gauze and tape for abrasions and two cravats for the shoulder or wrist, rigid material for a splint can usually be found lying around if the wrist is bad. If you plan on helping others, some rubber gloves in the case they are bleeding which can also be used to keep your hands clean on a tire change. Antiseptic cleansing wipes are good too.
However the best thing I think you can carry for emergencies is a cell phone.
Other than the rubber gloves & cell phone you will not find any of this on my bike.