When we started beekeeping, we got the doc to write for an epi-pen which we keep in our bee bag. Neither of us are allergic, but a small number of people do seemingly randomly become allergic after not showing reactions previously, and there's also a chance that a visitor could get stung. IMO it's a good thing for a trauma kit and if you've got insurance they're really cheap (and if you don't, CVS has a generic version 2-pack for $110).
I've also got some older (officially expired) tylenol with codeine in there. It may not be 100% as effective as when fresh, but it may be better than nothing...
I'm with you on the typical first aid kit being kinda pathetic. To me a band-aid isn't really first aid - I mean technically it is, but the truth is that the only reason I use band-aids is to keep a wound clean as I don't bleed much and after a bit of pressure bleeding isn't a concern - contact with the wound is; while it would be convenient not to have to stop work to go home to slap a bandaid on a little cut it's not a big deal - I'm more concerned about the trauma part of the trauma kit, where you may want to stabilize the problem right then and *then* go home.
I also have sports tape (white cloth, not slick) that's good for stabilizing; medical tape (white, heavy, slick) works very well as a blister protector or prevention (if you regularly get blisters in a spot, or feel irritated in a spot, put some tape on it).
If you think there may be someone else to help (help you, or you help them), you could consider adding an "onsite defibrillator"... $1-1.5k pretty pricey, but could be a lifesaver. Don't know if they "go bad" over time (other than presumed batteries).