Those are some really awesome ideas! I've voted for the ones I find most interesting.
I had a suggestion to make. The project I made in my final year of college was a visual search tool which could rank a given set of images based on their similarity (in terms of overall shape, color, pattern) to a query image.
I've, therefore, done a lot of poking around in the field of image similarity and have always thought that a good use for Google Plus's vast Image database would be to enable users to search 'visually', i.e., find images similar to a query image.
Tineye already does a decent job with this, but (as far as I know) it only finds exact matches and doesn't suggest images that're similar. Also, Google Image search already has this option so I think it would be great to have something like this rolled out images in Google+ as well.