and I must admit having not taken the time to read the entire (loooong) thread.
Anyway, here's some ideas to throw out for anyone interested:
Dravidians... people of southern India, and, in likelihood, of northern India prior to the influx of Indo-Aryans and later Iranians (Sakas) and Mughals (Mongols). Dravidians are also theorized to be the racial/linguistic stock of the ancient Elamites, who figured prominently in early Mesopotamian history (and were ultimately absorbed by the southwest Persians). There is also some theorizing that the Hadraumats of southern Arabia are Dravid in their origin (and not African). Anyway, these are a somewhat forgotten historical (and modern) population whose greatest contribution to civilization may have been buried in the ruins of Mohenjo Daro and Harrapa.
Anasazi... another lost people, these ancient Amerindians populated the four corners area of the American Southwest many thousands of years ago. Their society seems to have imploded and scattered with the denuding of their lands (reminiscent to some degree of Saharan and Easter Island crypto-histories). They are most notable for their haunting and (to my eyes) magnificent cliff-face cities - which you can still visit today. They never advanced beyond the stone age, but they are interesting.
Mississippi Mound Builders... like the Anasazi, these people are largely lost to history, as there are no written records of their existence, only their monumental earthworks. Personally, I'd incorporate this culture into the broader, semi-fictive framework of the "Dakota" civ, which would also include the earth-lodge, agrarian Mandans of the Missouri River.
Scyths (more broadly representing the various influential Iranian steppe peoples, such as Sakas, Sarmatians, Amazons, Roxolani, Alans, and possibly Cimmerians and Khazars)... the original horse lords of central Asia. Before the Turks, before the Mongols, these Iranian-speaking nomads (like those other steppe people) didn't so much create empires as they took over and engreatened the empires of others. Their list of control is impressive and includes empires of India, Anatolia, the Persian plateau, the Hungarian plains, and along the central Asian trade routes. The Iranian command of the steppe was for far longer and sustained as much or more influence than the subsequent Turks and Mongols (and they provided the cultural and racial substrate for later Turk, Russian, Mongol, and Gothic populations).
Malays (Indonesians)... I bet this has been said, but even if so should be repeated. The influence of Malay traders over southeast Asia is obscured by subsequent European traders.
Zimbabwe... something happened there (not much, granted).
Benin... this was a legitimate west African Kingdom that excerted some influence for a short time.
