The instances and complexity of ethnogenesis in North America is fascinating. For this answer I will dive into the history of the U.S. Southeast, but know there are plenty of other examples throughout North American history.

Around 1000CE complex chiefdoms arose throughout the eastern portion of North America, with the seat of power organised around large mound complexes. Cahokia, one of the earliest, largest, and most well known of these Mississippian chiefdoms, boasted over a hundred earthen mounds spread out over six square miles, and was estimated to be the home of more than twenty thousand people. Cahokia was part of the Early Mississippian period, and for the next seven centuries a cyclic pattern of consolidation, mound complex development, and then abandonment of the site characterized power ebbs and flows across the Southeast. Despite the rise and fall of various seats of power, archaeologists emphasize overall regional stability throughout this time. That stability would be challenged, then shattered after contact.

For the first few centuries following European contact the cyclic pattern of Mississippian mound complexes continued. Early Spanish explorers describe densely populated settlements surrounding monumental earthen mounds with micos (chief) able to control sufficient numbers of fighters to decimate the Narvaez entrada in Florida, and push Soto around the Southeast. While these early Spanish incursions might have destabilized a few chiefdoms, like Chicaza in Mississippi and Tascalusa in Alabama, stability continued, and some chiefdoms like the Caddo, Natchez, and Apalachee continued until the eighteenth century.

The arrival of English settlers in the South, and the emergence of the Native slave trade shattered the Southeast. While earlier scholars originally thought epidemic disease prompted societal collapse in the Southeast, we now know a combination of factors linked to the slave trade reshaped the region. Between 1685 and 1715 highly conservative estimates indicate the English, and their Native allies, enslaved 24,000-51,000 Indians and shipped them to plantations in the Caribbean. The slave trade set off a refugee crisis. Survivors tried to flee into the heart of the continent, displacing existing nations and sparking further conflict, and Spanish allied mission Indians fled south to the Keys where they begged for ship passage to Cuba to escape the slavers. Disease followed the slaving trails inland, prompting the first large verifiable smallpox epidemic in the late 1600s. A combination of slaving raids, warfare, disease, territorial displacement, and social upheaval shattered the Southeast.

From this shatterzone previously distinct populations began to consolidate and form coalescent societies. The Creeks, Cherokees, Chickasaws, and Choctaws grew in power as alliances of strength and convenience allowed formerly disperse groups to stand against European encroachment. These new groups would play the game of empires, forging alliances with the French or English or Spanish, uniting in common defense or fighting between themselves as needed, to maintain land, resources, and sovereignty. While the popular perception of Native American history depicts a static people, the story of the Southeast shows dynamic populations adapting, coalescing, and thriving to the impacts of colonialism. Ethnogenesis was one successful path of survival.

Sources:

Mapping the Mississippian Shatter Zone Ethridge and Shuck-Hall

Epidemics and Enslavement Kelton

Chiefdoms, Collapse, and Coalescence in the Early American South Beck

The Indian Slave Trade Gallay

This is a fascinating question. Thanks for asking it! I don't really know too much about the case you describe concerning the Franks or Langobards, so I can't speak to my examples being perfect versions of what happened with them. But I'll go off the description of ethnogenesis you're concerned with when you say "several groups of the same ethnicity, language and customs, possibly scattered tribes in origin...uniting in a greater entity speculated to have spanned from a loose confederation of tribes...to a more organized polity"

I'm mostly going to talk about one relatively well documented example of ethnogenesis the way you're describing it, applied to North America's indigenous peoples: the Haudenosaunee, or Iroquois.

Academics regularly use the term "ethnogenesis" when describing the Haudenosaunee. This polity developed from northern Iroquoian peoples who established a league in the middle 1450s. The original constituent tribes of the Haudenosaunee were the Mohawk, Cayuga, Seneca, Onondaga, and Oneida. In the 18th century, the Tuscarora joined as well. All of these groups had shared linguistic and cultural roots, but regularly fought and competed with one another prior to the league's creation. According to oral traditions, Haudenosaunee creation was spearheaded by three people: Dekanawida, Hiawatha, and Jikonhsaseh. The process saw member tribes establish a Grand Council which represented and shared disputes, goals, and decisions between the five (and later six) nations. The Haudenosaunee were an extremely important and successful sociopolitical group that played a consistent role in regional power struggles from the late 1400s until their political fragmentation after the American Revolution, where the group had sided with the British Empire.

Haudenosaunee ethnogenesis was "not restricted to a specific event or point in time marking Iroquoian origins, but rather continued throughout the precontact and contact periods, with cultural change accelerating during periods of coalescence, migration, population expansion, and incorporation" (Schillaci et al. 2017). Ethnogenesis is already a long and complicated process, and Haudenosaunee ethnogenesis largely occurred in a time and place that experienced some of the most radical transformations in human history. These Iroquois tribes united shortly after establishing maize agriculture, grew powerful in the context of European colonization, and then survived as a united entity in the face of U.S. oppression through the modern day. It is also certainly arguable that Haudenosaunee ethnogenesis is still an ongoing process. Its first steps happened in the 1450s, but many historians link the politically and diplomatically united Haudenosaunee entity to pressures and responses during European colonization. For example, Hart and Engelbrecht "argue that ethnic terms now in use for Iroquoian groups have their origin with the formation of population clusters in the sixteenth and early seventeenth centuries. Under this scenario, the ethnic landscape at the time of European contact was a recent accommodation, not a permanent one" (2012). Today, members of the Haudenosaunee constituent tribes continue to claim various mixed relationships with their specific tribe (such as the Mohawk) and with the wider Haudenosaunee group. And this is happening with Haudenosaunee lands and peoples divided between the sovereign nation states of the United States and Canada, which are often hostile to Haudenosaunee interests. As a quick recommendation, I'll suggest Audra Simpson's book Mohawk Interruptus: Political Life Across the Borders of Settler States for more on the topic. In any case, Haudenosaunee ethnogenesis is a long and complicated but uniquely documented process that began shortly before European colonization and continues to this day.

I want to end this response by saying that I'm confident there are many more examples of ethnogenesis in indigenous North America. I chose to speak about the Haudenosaunee at length because there is extensive research an documentation of Haudenosaunee ethnogenesis. But the Americas before European conflict were full of societies, empires, and states that often gained power by uniting different political and social groups. One of these states was the Aztec Empire - more accurately, the Triple Alliance - began from an alliance between ethnolinguistically related but initially politically separate Mexica groups in the Valley of Mexico. I'm not sure I would make the claim that there was a process of Mexica ethnogenesis, but nor would I throw out that claim immediately. The splintering and reforming of ethnic groups is a common characteristic in human history; I do not think I would claim that it is truly limited to any specific region or time. More accurately I would say that it is a descriptive term which can be applied to some types of social evolution that are found throughout humanity.

And I'd end by saying that indigenous peoples in the Americas are often going through forms of ethnogenesis in modern times. There are many reasons for this - survival in the face of settler colonial oppressions, decline of individual group population numbers, increased recognition about shared histories, among others. Consider the Maya, who historically existed as separate and competing but culturally connected polities that did not claim shared identity, yet now often do so. An article directly titled "Maya Ethnogenesis" looks at the mixed internal and external factors that drove this process amongst the many different peoples we usually call Mayan (Restall 2004). Many other articles do the same for the Maya, and they are only one of thousands of indigenous ethnic groups in the Americas.

​

Sources

Hart, J., & Brumbach, H. (2009).On pottery change and northern Iroquoian origins: An assessment from the Finger Lakes region of central New York. Journal of Anthropological Archaeology, 28(4), 367-381.

Hart, J., & Engelbrecht, W. (2012). Northern Iroquoian Ethnic Evolution: A Social Network Analysis. Journal of Archaeological Method and Theory, 19(2), 322-349.

Restall, M. (2004). Maya Ethnogenesis. Journal of Latin American Anthropology, 9(1), 64–89.

Schicalli, M. et al. (2017). Linguistic Clues to Iroquoian Prehistory. Journal of Anthropological Research, 73(3), 448-485.