This is a great idea. The watch history itself can be done using Invidious's watch history (right now if you're logged in and watch a video, it gets added to the Invidious watch history).

I think we could add a Continue watching section with recently watched, but unfinished videos. We can save this information locally on Roku.

This does turn into a problem where the Continue watching becomes a graveyard of videos you started but you did not find interesting. Some perhaps an expiry date to these videos can be useful (they go away after X days of not continue watching them) and an easy way to remove videos from the list would also need to be put in place

Right! One thing to consider with the expiration date approach is that the cache size would be bounded by the number of videos a user can watch (and not necessarily finish) within a certain expiration date. This can be acceptable if we're not really concerned with a bloated cache. If memory is of much concern, though, perhaps a watch history list with a fixed length may be appropriate. Spitballing here, but perhaps the watch history list length may be configurable in settings (i.e., save only the 5, 10, 20 most recently watched videos).