I note a few design flaws in most brass knuckles, as I've seen them in media and the occasional example or replica:
They restrict digital dexterity. By which I mean they basically restrict your fingers' movement while wearing them. This isn't tremendously important, but does limit their casual use (say, holding a pistol AND a brass knuckle, or using a keyboard etc).
Most of them seem to have square edges. I'm not sure if this is a deliberate design choice or just an affect of the casting process. The blunt-ish edge means that either your punches are off centre, or you're spreading out your point of impact.
Many of them "float" a distance from the fingers, resulting in a lot of slippage and force directed at the skin between your fingers (ow). Some designs I've seen have a palm grip that would push against your palm next to your thumb (apparently this is the heel, or thereabouts), which definitely would help.
Many of them waste a significant amount of mass and volume for the inclusion of the ring finger and the little finger, neither of which should be that involved with punching (a downward mallet smash could justify the inclusion of another force concentration device). This means that components that need to be stronger aren't as strong as they could be, and that the brass knuckles are less concealable than they could be.
The intended angle of impact is somewhere below the knuckles, meaning that impacts can cause more injury to the wrist (rotationally pulling the fist downwards). While it's a fairly small force, why take that added risk?
To resolve these problems, I have been thinking about the mark 2 fist, or the Enhanced Brawling Impact Device. The intent is it could be worn as a ring or sewn into a set of gloves (fingerless or no), thus preparing the wearing to be able to break bone with a well placed punch. I do have a bit of a penchant for flowing organic designs, which may introduce some inefficiencies. Alas.
The design is also intended to be usable with either hand, looping around the index and middle fingers, with the business side of the EBID lining up with the knuckle when a fist is made. The requirement to be usable when striking with the fist lateral does mean that it isn't as symmetrical as traditional brass knuckles are.
This could be made of any reasonably solid material, but if you use wood I'm not responsible for splinters. Metals have the obvious advantages of being fairly sturdy and packing a lot of mass. Padding may be included inside the rings so as to reduce the load on the wearer's fingers.
The inside of the rings should attempt to spread the force evenly over an area of the fingers, to ensure the safety of the wearer's fingers. The index and middle knuckles should be kept steady, not forced apart or together (though slightly together is preferable).
I haven't yet learned to make this sort of thing in zbrush yet, so here's some crappy sketches. Once in zbrush, I'll upload this and then you can 3D print it and pop some eyeballs.