JEDIT : This is now the non spoiler thread.
Plot spoilers should only be posted in this thread: http://originaltrilogy.com/topic/Rogue-One-Spoilers-Thread/id/49191
Teaser was shown at Celebration 2015. Can’t really make out much but here it is - https://m.youtube.com/watch?v=7n8i579d_JI
Jedit: a working link to the taken-down ‘For a thousand generations…’ video can be found here - www.dailymotion.com/video/x2nbjz3