Mordvins and Permians came the closest IOTL. Permians had their own alphabet and two of their own states . But IIRC Great Perm by that era was already fairly assimilated into Russian culture because it emerged as a vassal state of Novgorod and maintained a sort of independence by being at the edge of Novgorod's sphere of influence.
The Mordvins seemed like they were on the cusp of it in the early 13th century under their strong prince Purgaz, but they fell afoul of the Mongols. Purgaz was murdered by another leader, Puresh, who allied with the Mongols but then tried betraying them yet his plot was discovered and he and thousands of Mordvins were killed. There was an autonomous Mordvin portion of the Jochid state that became the Kasimov Khanate, but its rulers were Tatars IIRC and its people either converted to Islam and were assimilated into the Tatar ethnicity or converted to Christianity and became Russians.
Given these facts, the Mordvins (specifically the Erzya) seem like a better candidate than the Permians since they are at the borderland between Russian states and Volga Bulgaria (which later became the heartland of the Tatars) and could theoretically convert to Islam or Christianity (ideally something separate like Shia Islam or Nestorianism) and adopt an alphabet suitable for their language (like the St. Stephen of Perm's alphabet). They could emerge as a buffer state, conquer nearby Volga Finns like the Mari/Cheremis and play the Russians and Tatars off each other. Unfortunately, I think their lifespan is fairly limited unless the Jochids reform into a gunpowder empire and take the place of the Russians in Siberia. And that in turn leads to problems if Russia collapses but another power like Poland-Lithuania isn't able to take over.
As a side note, Medieval Russian sources also describe various Khanty and Mansi "principalities" but it seems like these were closer to tribal confederations than anything else and the Russians were describing them in a context they would understand (much as 16th/17th century Europeans described tribal confederations in North America as "kingdoms" and powerful native leaders as "kings").