Koyo peaks have yearly variations, however, I searched destinations with the average Koyo peaks in late October. Here is a list of the best three Koyo destinations by Jorudan in each prefecture in the area covered by JR East Tohoku Pass. Some are also listed in Zekkei Japan for October. Zekkei Japan has an entry for Hachimantai but as a snowy scene.
http://zekkeijapan.com/spot/search/?type=month&month=10Tsutanuma Pond Aomori 1 https://sp.jorudan.co.jp/leaf/spot_117749.html
Hirosaki Castle Aomori 2 https://sp.jorudan.co.jp/leaf/spot_54941.html
Mt Hakkoda Aomori 3 https://sp.jorudan.co.jp/leaf/spot_54940.html
Dakikaeri Keikoku Gorge Akita 1 https://sp.jorudan.co.jp/leaf/spot_54963.html
Kakunodate Akita 3 https://sp.jorudan.co.jp/leaf/spot_54964.html
Chusonji Temple Iwate 3 https://sp.jorudan.co.jp/leaf/spot_54952.html
Yamadera Risshakuji Temple Yamagata 1 https://sp.jorudan.co.jp/leaf/spot_54965.html
Nishi-Azuma Skyvalley Road Yamagata 2 https://sp.jorudan.co.jp/leaf/spot_54966.html
Mt Gassan Hanagasaline Road Yamagata 3 https://sp.jorudan.co.jp/leaf/spot_54968.html
Naruko Gorge Miyagi 1 https://sp.jorudan.co.jp/leaf/spot_54955.html
Akiu Waterfall Miyagi 2 https://sp.jorudan.co.jp/leaf/spot_54953.html
Zao Echoline Road Miyagi 3 https://sp.jorudan.co.jp/leaf/spot_54956.html
Goshikinuma Ponds Fukushima 2 https://sp.jorudan.co.jp/leaf/spot_54975.html
Mt Bandai Goldline Road Fukushima 3 https://sp.jorudan.co.jp/leaf/spot_61564.html
Ikaho Onsen Kajika Bridge Gunma 1 https://sp.jorudan.co.jp/leaf/spot_104810.html
Lake Haruna Gunma 2 https://sp.jorudan.co.jp/leaf/spot_69766.html
Mt Tanigawa, Tenjindaira Gunma 3 https://sp.jorudan.co.jp/leaf/spot_54994.html
Nikko Irohazaka Slopes Tochigi 1 https://sp.jorudan.co.jp/leaf/spot_54985.html
Lake Chuzenji Tochigi 3 https://sp.jorudan.co.jp/leaf/spot_54983.html
Mt Tsukuba Ibaraki 2 https://sp.jorudan.co.jp/leaf/spot_104805.html
Oku-Chihibu Nakatsu-Kyo Gorge Saitama 2 https://sp.jorudan.co.jp/leaf/spot_54995.html