The first trailer of Valerian and the City of a Thousand Planets (July 2017) will come out this week,. We've had teaser for Guardians Vol.2, Life, Pirates 5, Wonder Woman, Dunkirk.
I think a first teaser is for a very very near future.
The first teaser of Prometheus was illegally released on November 20. The end date of filming is more or less the same for Covenant, I think it's possible that something ready for us at that time.