Unlocked Nexus 6p stuck in boot loop
From what I understand, you said initially that the phone was unlocked and rooted, added to that, since you get the warning message during boot, the bootloader is also unlocked (evidence is the unlocked symbol that comes up at the bottom center of the screen). I used to run into this trouble while rolling back to stock and clearing userdata and cache before the first boot. What you can do again is:


Boot into fastboot.
fastboot erase userdata  [or fastboot format userdata]
fastboot erase cache [or fastboot format cache]
fastboot flash radio &lt;&lt;radio img&gt;&gt;
fastboot flash bootloader &lt;&lt;bootloader img&gt;&gt;
fastboot reboot-bootloader
fastboot flash recovery &lt;&lt;recovery img&gt;&gt;
fastboot reboot-bootloader
fastboot flash boot &lt;&lt;boot img&gt;&gt;
fastboot flash system &lt;&lt;system img&gt;&gt; [at this point, you must see
 it actually recreate the system partition]
fastboot flash userdata &lt;>
fastboot reboot-bootloader
volume up / down and get into recovery mode
Power button + vol up [then let go of vol up, then power] to enter
 recovery options
use vol up / down and format the device [I believe it's the last
 but one option]


Reboot device and sit tight for at least about 10 mins.