Tactical first aid is pretty simple; a bunch of gauze, a bunch of ace bandage and coban, wound wash, an assortment of band aids, medical tape, bacitracin, tourniquets, trauma scissors, forceps, hemostats, quick clot (if you have the budget for it) and whatever else you like.
When someone says "tactical First aid" that generally means gunshot and laceration wounds that cause major bleeding and soft tissue damage. Burns and broken bones can also happen but those injuries require more of a long term treatment. Basic splinting and pressure dressings are all you generally need for them.
Comfort items such as toilet paper, foot powder, sun block, bug repellent, pain killers and water treatment might be helpful too, depending on what type of bag you are building. Get all the basics first, then have fun and be creative.