YouTube does this for a lot of videos where you don't have to click into the video to see some of it. Not sure what would be required to make that happen but it would be nice to have for videos on this site as well.
That seems kind of unecessary since the filesize limit means there aren't going to be many long videos on the site. Not that I hate the idea. Not sure how compatible it would be with still being able to right-click and save the videos either, since it would require an interface overlay.
In case it wasn't clear, this would be for when you are viewing the "thumbnails" of videos, not on the video page itself. The idea being to get a better idea of what the video is before clicking and having to load another page.
I could see the value in it, especially for videos with an unappealing cover photo or thumbnail, such as:
Ah, I thought you meant the thumbnail previews when you're hovering over the seek bar on a playing video. I'm good with previewing videos from the video thumbnails, not sure how easy that would be to implement. Youtube autogenerates a 3-second webp animation for each video as the preview, from what I understand.