There was a rumour that they'd release something derived from the Nexus 9 with their own branding. Whether there was anything behind that, or just a piece of fairly simple speculation, I've no idea. Not seen anything more concrete lately.
Remember that 16GB is total installed storage, some of which will be allocated to the operating system, cache partition, etc. So in reality you will most likely have 10-12GB available from a 16GB device. If you don't install large apps (e.g. high-end games) or keep media on the device (video in particular eats space) that should still be enough. If HTC do release such a device it may have an SD card slot - if so that could be used for media, but don't count on it for apps.