Sorry if this is more of a history of an ATL Eastern Europe than an ethnic group, I got carried away. They're obviously based on the self-identified Yugoslavs although not just an Eastern European parallel.
Vostokoslavs
Ancestry: Eastern Slavs
Language: East Slavic languages, Neo-East Slavic [1]
Population: 1,000,000
Culture: Eastern European
Religion: Eastern Orthodoxy, Eastern Catholicism, minority irreligion and rodnovery
Region: Eastern Europe, United States, Canada, Australia
The Vostokoslavs are those Eastern Slavs who hold onto a united Eastern Slavic identity based on the supranational Federal Republic of Vostokoslavia instead of any individual Eastern Slavic identity. Its roots lay in 19th century Vostokoslavic pan-nationalism that saw the Russians, Novgorodians, Ukrainians, Galicians, and others as a component of a united Eastern Slavic nation, Vostokoslavia. The Vostokoslav identity remained a minority during the heyday of the Federal Republic of Vostokoslavia in the 20th century, representing the most diehard believers in the Vostokoslav ideal as well as those of mixed descent from various ethnic groups. The Vostokoslav identity was also prominent in nations abroad thanks to immigrants who identified with the nation their ancestors came from.
The origins of the Vostokoslavs lay in Kievan Rus, the medieval state of the East Slavs with its many principalities. The conquest of Kievan Rus by the Mongol Empire in the 13th century brought about what Vostokoslavs decry as the dark ages of East Slavic civilisation. From that point forth, the Eastern Slavic states became divided into numerous principalities subject to either the Mongols or by the 14th century, the Grand Duchy of Lithuania. Countless peasants were abducted by Mongol raiders and sold into slavery during these centuries while princes squabbled with each other over their right to receive recognition from the Mongol khan ruling the Ulus of Jochi (the last surviving portion of the Mongol Empire).
By the 17th century, new powers emerged to contest the domination of the Russian states. In the north, the Swedes, now firmly united behind their Danish monarch in the United Kingdoms of Scandinavia, returned to Eastern Slavic lands to contest their domination--by the late 17th century they fully subdued Novgorod, Pskov, and other northern principalities. In the middle, Lithuania, united with Poland in the increasingly centralised Polish-Lithuanian Commonwealth, continued their expansion into former Kievan Rus and upon defeating the Jochids set themselves up as rulers of all Russia. In the south, the Ottomans conquered Crimea from the declining Jochids and increased their slave raids and tribute demands. Despite many rebellions and attempts at holding off outsiders, by 1700, not a single East Slavic state maintained anything but local autonomy thanks to centuries of Jochid, Scandinavian, and Polish domination.
During the 19th century, the modern ethnicities in the region--Belorussians, Ruthenians [2], Russians, Ukrainians, and Novgorodians--crystallised, yet a sense of "Vostokoslavism" driven by romanticism of Kievan Rus' emerged with it. Intellectuals and agitators from these five ethnic groups demanded their own nation states in this time and as turmoil struck Scandinavia and Poland-Lithuania, began to receive them with the independence of Novgorod in 1815 and the Cossack State in 1878. These new states were hotbeds of militancy as they sought union with their ethnic kin and hosted independence activists from elsewhere in the region. It was in this setting that Vostokoslavism gained popularity, despite criticisms from a socialist, nationalist, or religious standpoint.
After the collapse of Poland-Lithuania in the Great War in 1919, the East Slavic portion of the Commonwealth decreed the formation of Vostokoslavia with its capital in Kiev as a means of putting down the regional communist uprising, quickly being joined by Novgorod and the Cossack State upon giving appropriate concessions to them. The ruling Prince of Novgorod was elected Tsar of this new state. Yet despite initial enthusiasm, a great deal of internal tension and external agitation caused instability in the state. Assassinations of monarchs and prime ministers, religious tension between Eastern Catholics and Orthodox Christians, socialist militancy, and above all ethnic tension exacerbated by regional underdevelopment and perceived neglect resulted in the evolution of Vostokoslavia into a loose confederation by the 1950s. Vostokoslavia fell apart in the 1980s after a series of mostly peaceful (besides the Polissian War between the new states of Ukraine and Belarus) secessions caused by decades of this tension as well as economic recession and the major nuclear accident outside of Kiev.
As a result, the Vostokoslav identity never truly developed among the masses--Vostokoslavs primarily identified by regional ethnic groups like "Russian" or "Ukrainian". However, distinct Vostokoslav culture existed from decades of being ruled from Kiev and a segment of intellectuals and nationalists kept the Vostokoslav dream alive into the 21st century through measures like promotion of the "Vostokoslav" language (a unified East Slavic language based on Old East Slavic) and promoting cooperation between the five nations that emerged from Vostokoslavia.
[1] - A conlang based on Old East Slavic, the language of Kievan Rus.
[2] - Separate identity from the Ukrainians TTL as they are more Polonised and their language is based on the Galician dialect