I love the idea, and back when I was repositioning the front door of my own house, I did the same thing. I made my time capsule from a PVC pipe, which I also capped at both ends with PVC. Just to make sure that the eventual find was not mistaken for debris, I tied a red ribbon around the capsule, making it look like an old-time scroll case.
A time capsule fires the imagination of people of all ages, so by all means include your family, neighbors and friends. The process can have a magical effect for all involved.
I included letters from family members, my immediate neighbors (who wrote down things I had never known about the neighborhood), and friends. The friend who installed the new door wrote a note. I also asked my friends to contribute small items of significance to themselves, and to talk about the symbolism.
Another friend, proud of her Celtic heritage, put in a brass rubbing inspired by the Book of Kells.