UPDATE: Devs will be on hand for the "Next Chapter" on March 21st at 1:00 pm Pacific / 4:00 pm Eastern in the ESO Twitch.tv channel.
The Elder Scrolls Online Twitter was updated late Friday with an image of a letter signed by Queen Ayrenn Arana Aldmeri that reads, "Your Queen has exciting news to share as soon as the time is right. I bid you watch and listen for the next herald, who shall present a royal announcement of great significance to us all."
Beyond that, there is little to indicate exactly what devs are teasing, though the Twitter thread is awash with speculation that this is a teaser for a Summerset DLC and / or something to do with the Psijic Order.
You may remember that in February, dataminers discovered a ton of information about a possible forthcoming DLC. This appears to be coming true. You can read what was uncovered here.
We don't know when more will be revealed, but we'll keep you posted.
Thanks for the tip, Iselin!