Also used by Google's Nexus 6, the BCM4356 supports Wi-Fi 802.11 a/b/g/n/ac and Bluetooth 4.1, and should significantly boost the Wi-Fi performance of the One (M9) over the One (M8).
Another thing that LlabTooFeR has to share is related to the rear camera of the M9. Reportedly, this might use a 20 MP Toshiba T4KA7 sensor (1/2.4-inch, 1.12-micron pixels) - which, of course, is expected to outperform the two camera sensors found on the back of the One (M8).
As you may have heard, HTC will announce the One (M9) on March 1 at MWC 2015, a couple of hours before Samsung should unveil its Galaxy S6 and S Edge. A One (M9) Plus version might be unveiled at the same time. While the (M9) Plus could sport a 5.2-inch Quad HD display, the regular (M9) is expected to have a 5-inch 1080p screen. Of course, we'll know for sure if that's true only after HTC officially reveals all the details.
source: @LlabTooFeR (1), (2)