Originally Posted by Two Wheeled 'Tard
Let's be honest, 99% of the things that you might accidentally leave at home, you can easily buy on the road.
I don't know what your plan is, but I wish I'd brought a water purifier on my first big road trip. I tend to seek out very remote places where potable water isn't easy to find, and having a little pump backpacking filter would have made my life much easier.
That's been my experience as well. I started of things that I wish I had and couldn't really think of anything. If something is really useful in some part o the world, there will be plenty of local sellers selling it. It might be overpriced and it might be very low quality, but it will be available.
The one thing I did think about is the water bottle purifier you are talking about. I wish I had that in some more remote areas. If you did your research on this, please post as to what you think is decent. There seem to be many options.