The next Android version that Google previewed today as “L preview” or “L release.
What HTC have said so far is
…Our approach will be the same with the Android L release from Google. This release promises to bring new features and tools to Android and we are excited to bring it to you and your HTC. As part of our HTC Advantage commitment globally, we pledge to update all of our HTC One (M8) and HTC One (M7) smartphones within 90 days of receiving final software from Google.
We take Android updates seriously. With #HTCAdvantage, we pledge to have the L release on your (M7) and (M8) device 90 days after Google’s code release. Full details: http://s.htc.com/1pOLQig
I will of course keep you updated in this, will post as soon as I know something else/more.