Off the shelf first aid kits are... problematic. Either they're so simple and bare-bones to be useless for much more than cuts and scrapes, or they border on a field surgical kit. There's a massive gulf in between, and that gulf is where some of the best first aid kits fall, but to get there you pretty much have to build your own.
So what considerations and decision criteria do you apply when you build a kit?
Space: How are you carrying and storing your kit? A kit meant to fit in your pocket is a far different build than a kit for your car or house.
Intended use: What general purpose are you gearing for? Are you building a kit for a work gang on a construction site? Do you spend a lot of time camping and hiking? Are you alone a lot, with the need to treat yourself?
Level of training: How much medical background and skill do you have? You should know how to use every piece of gear you carry, and I don't advocate carrying gear you don't know how to use. As an example, I have zero training in sutures, and very minimal and passing instruction on deflating chests, so carrying kits for that (commonly found in those nigh-surgical kits) is wasted space, weight, and money for me.
Personal needs: If you or a loved one has a medical condition with specific needs, by all means, you have to plan for that.
The answers to these questions will direct how you build and store your kit. As we proceed, please make suggestions or offer substitutions, as I guarantee none of us have seen every gizmo and new first aid toy out there. If you have any questions, please comment or email.