Todd makes some good points. A pistol light is not the place to show off fancy modes or excessive lumens. It needs to work, it needs to work consistently, and it needs to be simple.
The switch needs to be big enough to work with gloves, and not need a lot of dexterity or pressure to operate. Unless you have a really, really good reason, stick with the standard momentary if you push one way, and constant lock the other.
IMHO, a very nice, well defined center spot and somewhat dimmer flood is a good answer. A nice, medium level flood is good for situational awareness, and a small hot center can be a good ‘rough guess’ about point of impact.
Regarding brightness, this is not a searchlight. It is primarily a ‘social distance’ light. Being too bright, reflections can interfere with your ability to see things that are not directly in front of the light. I’m somewhat OK with a manually engaged HI mode. The primary on/off is for up close and personal. If what you’re shooting at is further away, you can take a second to bring up the brightness. I’d see this more as a long-gun mode, but it may have application for duty pistols. .
I just realized that I’ve been thinking exclusively in the context of a handgun. If you want something to live on a rifle or carbine, you want to lean more towards throw. You’d definitely want a near/floody mode and a far/spot mode. How to do this - I don’t know.
Power - CR123 is still the king of weaponlights. All weather and readily available. I’ve got a small pistol light with an integrated battery that charges with a magnetic connector. I’m mostly OK with it for a home or civvy carry gun. If it was on a duty gun I’d have issues with the limited run time and not being able to swap batteries in the field.
If you really want a differentiator, offer it in mixed colors. White/IR, or White/Red. How to change modes in a way that is simple, small enough for a pistol light, and really, really hard to screw up is the challenge. If you sell into the high speed / low drag market, someone accidentally engaging White instead of IR will earn you enemies. If you want to do this on a rifle light, check out the Vampire line from Surefire.