I think I heard that Esav and Yaakov were supposed to be like Yissachar and Zevulun; partnering together in Avodas HaShem. So then would both have become our Avos? I'm interested in the earliest sources anyone has on this topic. Thank you very much!
Here's an interesting source (hint) why Eisov is considered [potentially] a forefather:
The Pesach Hagaddah (מתחילה עע"ז היו אבותינו):
מִתְּחִלָּה עוֹבְדֵי עֲבוֹדָה זָרָה הָיוּ אֲבוֹתֵינוּ, ...
וָאֶקַּח אֶת־אֲבִיכֶם אֶת־אַבְרָהָם מֵעֵבֶר הַנָּהָר
וָאוֹלֵךְ אוֹתוֹ בְּכָל־אֶרֶץ כְּנָעַן,
וָאַרְבֶּה אֶת־זַרְעוֹ וָאֶתֵּן לוֹ אֶת־יִצְחָק, וָאֶתֵּן לְיִצְחָק אֶת־יַעֲקֹב וְאֶת־עֵשָׂו.
And I took your father, Avraham from over the river and I made him walk in all the land of Canaan and I increased his seed and I gave him Yitzchak. And I gave to Yitzchak, Ya'akov, and Esav.
If you notice, regarding Avrohom only Yitzhak is mentioned not Yishmael, but for Yitzhak both Eisov and Yaakov are mentioned, hinting on the fact that originally Eisov [had a potential] to be [one of] our forefather.
I can't provide the sources others than the teachings of AriZ"L that speak of this idea.
This idea can be explained in many ways, but to put it simply, for the Jewish nation there's a body and a soul, Eisov destined to be the body and Yaakov the soul. Similarly to Zvulun and Issachar - one is busy with the earthy stuff and the other with the heavenly. So basically he was destined to be one of our forefathers.
While Yaakov was איש תם יושב אהלים, spending all his time studying Torah, Itzhak preferred Eisov's was that was more challenging and that was the reason for ויאהב יצחק את עשיו כי ציד בפיו.
Year after year Eisov started to fail his mission and Rivkah was the first to realize that he's not gonna make it and be [one of the] foundation of the emerging Jewish nation and Yaakov should take his job. G-d agreed and Yaakov wen abroad and married the two sisters - one for himself and one for Eisov etc.
That's the reason the Gemmorah calls Eisov ישראל מומר a converted Jew and his head is buried in the Cave of the Patriarchs.