I would consider the target diver.
Tech divers have fairly common standards they adhere to, for example, carrying their backup lights attached to harness D-rings using bolt snap attached with cave line at the tail and an EPDM band at the head. The light will snug up against the harness. In this case, the light only needs a suitably sized hole at the bottom to allow several wraps of cave line. The housing shape and material can also help - I found some lights too slippery to rest nicely against the harness strap. The EPDM band rides up on the material and the head gets loose. A recreational diver would probably be just as happy with a lanyard passing through the same hole. They would likely store their backup light in a pocket and maybe add a clip to the lanyard to prevent losing it. A good design could satisfy both tech and rec divers.
Lights like the L&M Gobe are annoying because they don’t provide this basic attachment point that is tech diver friendly. The Gobe has a thin lanyard strap but its attachment hole is too small to allow cave line to attach a bolt snap. The on/off button shape and placement where the EPDM band would be also makes it a liability for being turned on accidentally underwater without the diver knowing so it is not a good tech backup light.
Head mounting is an option used by many side mount divers. Many will just improvise their own mounting with bungees, I expect, or if you offer some sort of clip to a head mount strap, perhaps they might use that as a basis.
For the primary light, tech divers will gravitate to the hard Goodman handle. Also understand that there are also three states a light will be in: permanent stow, in-use, and temporary stow. Facilitating the permanent stow and temporary stow positions by allowing places to attach bolt snaps to the Goodman handle and to the back of the light will gain bonus marks. Rec divers might gravitate to a soft Goodman handle.
For primary lights, the permanent stow is more a function of the Goodman handle. Thus if you provide a light that has options for mounting to different types of handles or a quick release, and your Goodman handle has a hole to attach a bolt snap with cave line near (typically above) the thumb, you should be good. The permanent stow just means the light is secured in an off position on the D-ring and not getting in the way.
The temporary stow position is often just a loop of bungee that allows the tech diver to secure the light with a double ended bolt snap to their D-ring so the light faces downwards during a dive, allowing them to use both hands. My canister primary light has a groove in the cable strain relief behind the light head to allow this bungee loop to be attached. It is fairly simple for the diver to add the loop with a cable tie, cave line or other solution if there is a light cord. If there isn’t a cord, then a suitable rear mounting hole, just like with the backup light, would be appreciated.
The tech diver doesn’t really ask for much other than good basics: sensibly placed mounting holes through which they will normally attach snaps using cave line, a primary light that will work with a Goodman handle, a beam pattern useful for signaling, and a switch mechanism that is robust and foolproof and will not accidentally turn on and exhaust the batteries, and (for some) use with standard non-rechargeable batteries. I do believe that these requirements will just as easily be enjoyed by rec divers.