I would love to see a way to let users and site visitors sort/filter gallery images by tags. This would be useful for just about anyone who uses galleries as a portfolio, product offering, travel log — basically any time you have more than a few images and more than one subject or topic.
There are some work-arounds floating around, but none of them work directly with the gallery feature at the point of adding images right in the Build menu (others require you to add code, images, and labels on the backend).
I am always adding new images to my galleries and being able to add tags to each and create sortable categories would really help visitors find what they are looking for. I know there are some options for this in the store features, but I'm just talking about regular galleries outside the store.