HTC has made the a Nexus phone before back in 2010. Ever since, the task has fallen to Samsung (with the Nexus S), LG (with the Nexus 4), Motorola (with the Nexus 6), and Huawei (with the Nexus 6P).
Google uses the Nexus range to give other Android manufacturers ideas about the kind of design and technological improvements that should make it into phones. The software on the Nexus is "pure" Android, without any third-party customisation.
According to @LlabTooFeR, a Twitter account that leaks Android news, HTC is going to make two devices: The T50, which could mean a 5-inch screen, and the T55, which could signal a 5.5-inch screen. For comparison, the iPhone 6s Plus has a 5.5-inch screen.
It's unclear how Google chooses its partners. LG has made the most Nexus handsets (3), followed by Samsung (2), Motorola (1), Huawei (1), and HTC (1).
The Nexus 5X and 6P, introduced late last year, came into two sizes and were made by LG and Huawei respectively.