I strongly believe Dropbox's video player should support adding caption files (e.g. .SRT, .VTT, etc.) to videos stored in Dropbox.
There are two reasons for why I feel strongly about this:
1) Google Drive users can do this, but I do not generally like Google Drive as much as Dropbox.
2) I am a paid subscriber of a 2TB account, an amount that vastly exceeds the storage capacity of my Vimeo account. For videos larger than 5GB, I would like to be able to use Dropbox not just for file sharing, but also for quality control and previewing purposes.
This would greatly improve Dropbox's functionality within the media and post production industries.