I'll limit myself to answer only a few of your questions for now...
Gauls
The Gauls were conquered and integrated into the Roman society, resulting in a Gallo-Roman culture speaking a combination of Celtic and Latin, which later contributed to the forming of the Romanic language of the Franks after Clovis. The Celts of most of France were integrated into the Frankish culture, and a whole lot of intermarriage took place (Frenchmen from the south of France have a lot of Celtic, Visigothic and even Basque blood in them!), so that the Celtic blood of Gaul dissapeared, lingering on in such backwaters as Britanny.
Germans
The different Germanic peoples as we know them (Franks, Friesians, Rugians, Visi- and Ostrogoths, Lombards, etc.) are actually different older tribes (Batavians, Chatti, Marcomanni, Quadi, etc.) formed into 'super-tribes' encompassing a variety of Germanic tribes once seen as independent.
For instance, two of the more important tribes of the Frankish confederation were the Batavians and the Chatti (the latter of Teutoburger [in]famosity). The Marcomanni and Quadi were both Suevian peoples ('Suebi' was actually a term which was used differently by the Romans; sometimes to designate the Swabian tribes exclusively and sometimes all the Germanic peoples), who had reached the status of 'super-tribes' in the 2nd century AD already, but later the Suebian tribes merged into the Swabian super-tribe, of which part remained in modern southwestern Germany, Switzerland and Austria, and part migrated to Portugal and northwestern Spain.
So: the Germanic peoples as we know them did not spring out of nowhere, but were simply products of the pressure from Eastern Europe (the Huns pushing Germanic peoples West) and overpopulation, prompting different tribes to band together and migrate to new areas.
Parthians
The Parthians remained until the 3rd century AD, when they were defeated by a revived Persian culture under the Sassanid dynasty. This dynasty banished all Roman, Greek and Parthian (Scythian) influence from their empire, and tried to emulate the ancient Achaemenid Persia, defeated by Alexander the Great, and sporting Cyrus the Great and Darius the Great as their greatest rulers.
The Sassanids took the Parthian way of waging war and made a professional army out of it (something the Parthians did not possess), and fared quite well versus Rome (something the Parthians could not say of themselves either). Since the Sassanid dynasty wanted the Persian culture to be the sole culture in Iran, its religion was Zoroastranism. This led to religious conflict and war with Christian Rome and Byzantium.
The Sassanids dissapeared from the stage of world politics when, after a long series of wars against the Byzantine empire in the 7th century, they were soundly defeated (crushed is more like it) by the Byzantine emperor Heraklios, in an amazing campaign. Previously, they had conquered all of Asian Byzantium, and captured the Holy Cross. When the Muslims began their conquests, both empires, tired from their long struggles against each other, were rocked to their cores. But Sassanid Persia, torn apart by a dynastic struggle after the death of Khusrau II at the hands of the Byzantines, led by a boy-king, and enormously weakened economically and militarily, was the one that fell before the Muslim onslaught under the leadership of the great Muslim general Khalid bin Walid.
Carthaginians
Carthage was razed and salt sown over its fields when Scipio the Younger captured it after the Third Punic War. Rome did all it could to eliminate the Punic culture from history.
However, after some time, Carthage was rebuilt by the Romans, and once again became a prospering port city (it did, after all, have a superb stragetical position). Another of the important cities in the old Carthaginian empire, Leptis Magna, produced an emperor: Septimius Severus.
Carthage was captured by the Vandals in the 5th century AD. Belisarius retook it for Justinian the Great in the 6th century, ending the North-African Vandal kingdom.
I don't know what happened when it was taken by the Muslims in the 7th century (after having held out for forty years longer than the surrounding area), but apparently the center of urban life moved the very nearby Tunis afterwards.
Numidians
The Numidians were, IIRC, Berbers. As you well know, many of the inhabitants of Northern Africa are either Berber, largely Berber, or have a respectable number of Berbers as ancestors.
When the Ummayad Caliphate sent an army to Spain to see if it was worth conquering, most of it was composed of Berbers (at least 66%).
When the last remaining member of the Ummayad dynasty died in Al-Andalus, the dynasty controlling Muslim Spain was followed by two famous Berber ones: the Almoravids and the Almohads.
Egypt
I don't know what happened to the ethnic ancient Egyptians, but what I do know, is this:
The Ptolemaic Empire and its culture were, as so many others, integrated into the Roman empire and culture. It remained a prosperous and peaceful province for by far most of the duration of the Roman empire.
However, in the 7th century the province of Aegyptus was lost by the Byzantines to the onslaught of the Muslims. My belief is that the nearness of Egypt to Arabia caused many Arabs to migrate there, and as you are sure to know, many of Egypt's inhabitants have a lot of Arab blood in them. The Berbers are a large minority in modern-day Egypt.
Dacians
Modern-day Rumanians are largely Dacian. Vlad the Impaler (Dracul) was a Dacian.
You see, the Dacians were never as Romanized as other provinces incorporated into the empire. This was due to the relatively short period that the province was under Imperial control (compared to Gaul, for instance). Also, the Dacians never intermingled with the different Germanic peoples passing through and living in Dacia as othe regions, it seems.
There is a lot of Slavic blood intermingled with the Dacian, but still the Rumanians of today can proudly say that they can trace a straight line from Burebista's realm, through persons such as Vlad Tepes, to the Rumania (and Moldavia!) of today.
Scythians
The Scythians of Europe (and therefore RTW) were pushed to the West by the Sarmatians, and became the final victims of one of the periodic domino movements of the steppe. When they were finally pushed out of their last ancient homelands by the Sarmatians, most of them fled south, across the Danube, into Phrygia, where they laid waste to the Phrygian (another branch of the Thracian peoples, just like the Dacians) culture there, and then apparently settled down and blended into the local population.
~Wiz
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