As der_saeufer says, I wouldn't tie it to a bottle.
Instead, make a few different versions--same electronics pack, but threaded for common bottle sizes; or else one electronics pack that screws into adapters for common bottle sizes. Plenty of people make good bottles, don't reinvent the wheel.
Put a window in it so the user can confirm that it's actually working.
Allow it to be powered by USB.
These days I would have little use for it but that's subject to change--long ago I certainly would have bought a UV sterilizer had they existed then and it's certainly possible our travels will once again take us into areas where I would like to have one. While I do hike I live in a desert--I do not own anything for gathering/filtering/sterilizing water because I do nothing where I can count on finding water other than during snowmelt and even then I have yet to encounter collectable water more than 90 minutes from the trailhead.