They basically already do with the first look stuff.
https://www.youtube.com/watch?v=Pb1JcO645BA
But as for an "official trailer". Not till a few things happen.
When they have plenty of different skills to show off.
When they have a some what complete combat to show off.
When they have an ETA on release (at least the season and year of release).
Plain and simple till they have something that will grab the attention of a gamer that isn't already interested there's no point in doing a trailer. We discussed it quite a bit in discord actually.