HTC is teasing an announcement on its Taiwanese Facebook page for a March 20 announcement, coinciding with the spring equinox.
HTC will give you an unexpected surprise
While it's possible that the announcement will be a phone, there's almost no chance that it will be a worldwide release, and it's definitely not the rumored HTC 10 successor that is expected to be announced and released sometime in the second quarter.
What's more likely is that it is the successor to the HTC One X9, rumored to be called the One X10. We've already seen leaked photos of the Europe and Asia-only device, and its announcement is already overdue: it was expected in January or February.
The phone will sport a modest spec sheet, including a 5.5-inch 1080p display, 3GB of RAM, 32GB of internal storage and a MediaTek P10 processor.
In the meantime, HTC's follow-up to the HTC 10 is expected to rock this year's latest spec sheet, including a Snapdragon 835, which may end up delaying its announcement and release until sometime in May or June.