Side by side would be great. I don't carry my lasers with me at all due to the shape of them. Either a side by side configuration or perhaps powered by a single CR-123a (if they could handle rechargeables that would be a huuuge plus). Continous operation (with a constant on switch. Click for on, click again for off, like a normal flashlight) without cool down would be nice aswell but I don't know if that's possible in this configuration.
Perhaps incorporate some kind of lens protection so you could stick it in your pocket without worrying about dirt making the beam speckled?
Personally I think I'd really only be interested in <5mW units. I have a 20+ unit and I'm kinda afraid to use it due to the potential eye damage risks. I have goggles but with them you can't really see anything so where's the fun in that
Oh and all this should be cheap of course