Beyond the 3rd and the 4th Century,Roman Empire was repeatedly bombarded with Germanic Invaders and maybe some of Non-Germanic origin in Eastern Europe as well. In this timeline,instead of fighting them,Rome decides to negotiate with them. The lands of Germanic peoples,Balto-Slavic people and the Scythians are incorporated into the Empire. Instead,they are given good power to rule and build up their own Romes all of which would be represented to the Greater Rome in Italy. Power sharing agreements are signed off for this huge territories which now extends up to Central Asia and Scandinavia. A Mega Roman Empire with a new Golden Age is born. Population increases steadily. Rome is now seeking treaties/agreements with Persia who now,will not dare to attack Rome which is so strong. The Empire is multilingual with Germanic,Slavic,Scythian,Middle Eastern,Greek,Semitic,Coptic,Caucasian,Celtic,etc. People interested learn others languages. Nobody in Europe and Roman Empire is treated as Barbarian. A new Monotheistic religion is formed taking all attractive elements of all the religions like how Zoroastrianism was formed and this would be an ethnic cum Universalizing religion with principles of flourishing ones own ethnicity(European/Caucasian) first but should welcome others willing to assimilate very well. Now,how would Tocharians who lived at the easternmost edge of Europe near Central Asia initially react to new settlers who look like them but speak a language like those they had never heard of(Germanic North/West/East,Latin,etc) till now? I dont think any Augustus like Emperor would not desire annexing Tarim Basin and parts of West China as well. So eventually,there might be a Tocharian dynasty and Western Iranian Dynasties as well and their mythology elements would enter the religion as well. So how would this successful Empire go into modern days? An advanced Greater Europe and the ME? Earlier colonization of Americas? Colonization of Eurasian steppes? How would the Modern world look like?