Guys, good news! It looks like Dmitry Grinberg is working on a Marshmallow port for the Nexus One! If you don't know who Dmitry Grinberg is, he is the hacker who works at Google and he ported Marshmallow to the Nexus 4 and the Nexus 7 2012. Other than these two ancient devices, he was also able to make Android M run on the Nexus S.
From his posting on his website, he is now focusing on the Nexus One!
http://dmitry.gr/index.php?r=06. Thoughts&proj=05. Android M on Nexus S
From his posting on his website, he is now focusing on the Nexus One!
http://dmitry.gr/index.php?r=06. Thoughts&proj=05. Android M on Nexus S