The Tamils. The Balts. The Kushans. The Scythians, Sarmatians, and Saka. The Silk Road. The Arawaks. The Nubians. Anatolians before the Turks and Byzantines. The Yoruba. The Tarascans. The Somalis. The Wolof. The Caucasus.
All of these. The Caucasus struck me in particular - how many timelines are there of alternate paths for Georgia or Armenia? I'd love to see a timeline where these groups and others, such as the Circassians, Chechens, and Ossetians, resisted Russian domination. Or a different religious mix - Muslim Armenia for example, or large parts of Dagestan and the North Caucasus retaining their ancient religions into modern times. Maybe a different pattern of migration - instead of Caucasian Turkic groups like the Karachay-Balkars and the Nogai, perhaps Uralic-speaking peoples migrate into the region. Or keep Azerbaijan Iranian-speaking and Zoroastrian, rather than Turkic and Muslim.
I'd also like to see more timelines based on Native North America (my field of study as a linguist). There's a lot of discussion on how the Natives could have resisted European domination, from various starting points, but not many scenarios have been expanded into timelines. Two ideas I had that I'd like to explore (whenever I finally finish my dissertation and thus earn back my free time) is a unified Native state in California or the Pacific Northwest successfully resisting white settlement, and a large-scale Native uprising in the United States during World War I.