Adding Videos as post-thumbnails (no stills, just the player in a smaller size)

You could make a plugin which adds a meta box in admin, where you can add a link to a youtube/vimeo video. The plugin should then save the url as meta data for that article when the article is saved.

When listing the articles (or viewing a single one), check if the meta data value is set. If it is, then output the corresponding embed code instead of the post-thumbnail.