If you like to root and mod your device then you already know that unlocking your bootloader normally means voiding your warranty. With the HTC One A9 HTC made the decision to make their devices developer friendly. It is their policy to not void your warranty for bootloader unlocking your device. When the HTC 10 ships you should be able to log into your HTCdev account and quickly and easily bootloader unlock your device. Once your device is bootloader unlocked you should be able to flash TWRP, root, and eventually rom your device! This policy is good for carrier unlocked devices purchased directly from HTC. More than likely your carrier branded HTC 10, especially Verizon or AT&T, will not be bootloader unlockable. You will have to rely on the some sort of vulnerability to gain root access to your device.