When it comes to flagship smartphones companies tend to keep to a particular naming scheme for the lineup. Samsung for example has been using the Galaxy S moniker for their flagship devices for the past six years, and they're set to go into 2016 with the seventh model in the series. HTC has been using the One lineup, with the last three flagship phones being the One M7, the One M8, and the One M9. It would only come across as reasonable to think that HTC might end up continuing to use the "One M" name for the devices that head up their lineup of smartphones, but a recent rumor suggests otherwise.
According to leaker @LlabTooFer, HTC will not be using the M name for the flagship phones anymore. Rather, it isn't stated that the M won't be the official name, but that the next flagship won't be the HTC O2 which was a previously rumored title for the device. Instead, HTC's next flagship is to be called the HTC Perfume. Beyond this, the rumor also suggests that the HTC O2 will no longer continue development and that HTC is done with the device, stating that the phone "is dead." HTC O2 was previously rumored to be the codename for the next flagship, and with "perfume" said to be taking its place, it's also very possible that perfume is also just an internal codename.
Having said that, HTC could still end up naming their next flagship as part of the current M line once the phone is finalized and begins production. In addition to the rumored name, it's also stated that the HTC Perfume will feature the next version of Android, Android 6.1, and that it will be sporting HTC's next generation UI skin, Sense 8.0. It's still fairly early to be speculating on a phone which likely won't launch until the Spring of 2016, but with CES and Mobile World Congress both on the way, there is a decent chance that more details will surface which reveal more about the upcoming phone. Whether or not HTC keeps the name perfume or if it's even a name for their device is still unconfirmed at this point.