I think having project oriented interface would be extremely convenient for subbers. This would mean having "Create New Project", "Open Project", "Save Project", "Export Project", "Add media to project" features which would allow subbers to designate working folders and other relevant variables. My understanding of the subbing process is that media tends to be grouped (e.g. series, seasons, trilogies etc) so having this sort of organization would be extremely appropriate.
That's actually a very good feature request! I will be sure to be keeping this request open for the future, however, in my humble opinion, priority right now should be given to getting the program to a fully-working state and supporting more subtitle formats rather than just SubRip.
However, this feature, should it be implemented would allow for subtitle projects to be easily accessible from inside OSE, such as multiple files in the same folder (episodes from the same series, like you said), automatic loading of the corresponding video for each subtitle file, changing between episodes with a keyboard shortcut, etc.
Great idea really, and subtitle projects should really enhance the need for a subtitle translation feature to be added soon.