For using Dropbox as a special remote, there are currently two choices:
- rclone supports the Dropbox API,
so the rclone special remote can be used.
(Actively maintained) - Alternatively, there is a dedicated special remote,
https://github.com/TobiasTheViking/dropboxannex
(Last updated 2014)
At this time it's not clear which is better, so if you find one works best, please comment below.