I actually don't think changing the video play button would be a good
idea. Its important that users realize the file is a playable video.
Having a prominent play icon is consistent with other sites (e.g.
youtube), although arguably most other sites aren't using the
thumbnail to also illustrate an article. If we were to do something
along these lines, I would think having it be a non-default mode would
be best, where the users could toggle discrete play button mode where
the play button goes into the bottom corner or something.