Hard Requirements:

These are the things that are absolutely necessary for this to be a usable product.

Entirely self contained within the crutch handle

The isn't meant to be a light that attaches to the crutches. It is meant to be, as much as possible, an integral part of the crutches that can be installed and forgotten about until needed.

This means that all parts of the light must be contained in the handle cavity:

• Optics
• Emitter
• Switch
• Charging connector
• Driver
• Battery

Obviously things like the optics and switch might protrude from the front a reasonable amount, but the vast majority of everything should be inside the handle.

Rechargeable via USB

An obvious choice, really, since you'll almost always been in arms-reach of a USB charger.

There is still a choice to be made about the connector, Micro-USB vs USB-type-c vs Lightning, but they are almost all the same physical size so the final decision can come later.

Does not need to be removed to be recharged

Goes with the USB-charging option. This just means that the charging connector must be outward-facing so the batteries can be charged while installed.

Water resistant

Since this is meant to be installed and then left there, it needs to handle getting a little bit wet. We're looking for water resistance, not waterproofing. Rain, not submersion.

That means the water resistance need to apply only to the exterior components: optics/led, charging connector, switch,  and a seal/gasket around the handle opening.

For later versions it may be possible to have the entire unit be sealed with something like a conformal coating or a completely sealed housing. 

This will be hard for the initial versions, so it may have to slide in the prototypes until later.

Single button operation

The target user may already have challenges with their finger dexterity, so the interface needs to be as simple as possible. Ideally, this means one button.

Capable of 100 lm at 60°

This is really a judgement call, based on most of the "consumer" flashlights I see having ~100lm at about 60 degrees.

Capable of running for 30 minutes continuously

People are going to be using these to commute, so they must handle decent work cycle.

30 minutes is also a judgement call, and it's probably a bit high for average daily use, but I would prefer if it was over-specified than under.

Depending on the emitter used, heat could be a concern. Couple the need for a heat sink with the need for water resistance and you have a unique challenge.

Soft (future, optional) requirements:

Things that would be nice for a v2.0 or a "plus" version, but not required for successful MVP.

Can be used as a USB power bank

Since you'll have a ruddy great rechargeable cell/battery anyway, why allow it to be used as a USB power bank too?

Battery charge state indicators

Would be nice to have discrete indicators to show the state of the battery. Of course, we have a big one already so there are options to just use some flash-pattern on that too.

Multiple modes

Adjustable brightness may be wanted and, as much as I don't like them, options for a flashing pattern may be of use to some people.

Adjustable beam angle

Everyone is not the same height and they won't use their crutches in the same way, so a way to adjust the beam able may be useful.

Overheat protection

Sense the temperature of the LED module and reduce the brightness if it gets too hot.

Head-in-the-clouds features:

Things that "would be really cool" but are probably too difficult or too niche to add.

Automatically turn off if the light is on but movement has not been detected in a certain period of time

With an accelerometer onboard, the driver could automatically detect of the light has been accidentally left on but is not in use. Ex: the crutch is on its side and no movement is detected for nnn time, automatically turn off light (or dim it?).

Automatically adjusting beam angle

Just like auto-leveling headlights in a car, make the beam angle adjustment be automatic. If you're going up a slope, the angle would be automatically adjusted, etc.

Adjustable focus 

Adjustable throw/focus, manual or automatic.

Full IPX waterproofing

Make the entire unit self-contained so it could be fully immersible and waterproof.