I am not talking about 6 Mio people. And i dont know how many balts there were. but i think there must have been more than 100.000.
according to this site:
Gaul and Rhineland had 5 Mio people around 300 AD. So my guess is that southern gaul would have 2-3 mio people. then not using a virus, but maybe a famine. and this famine could easily result because of the romans empire collaps, wealth is suddenly dropping, there are revolts, there is a civil war in rome, a civil war costs lifes., wars (gauls want to conquer southern gaul back, invading german barbarians fighting). So lets say we got to reduce gauls population to 3 mio. southern gaul would have a max. population of 1,5 mio people.
Lets say we have about 300.000 Balts, maybe 130.000 warriors, fleeing towards south, again there could be various reasons, there could be invading nomadic tribes, maybe germanic tribes or slavic tribes drive them towards south and force them to move away. they settle down in modern hungary around the balaton.
now we have about 300.000 people living here. Raetia Noricum and Pannonia together had 3 mio people in 300 AD, lets say pannonia has 200.000. In OTL when the magyars arrived there were about 200.000 people dwelling in pannonia. about 150.000 people seems realistically in this. the balts dont go and conquer whole of hungary, they just have their realm around Balaton (i assume balaton was rather sparsely populated, because of the swamps and marshes). Since there is no real powering empire in this timeline, various barbarian wars and migrations are much longer here, barbarians wander around without being smashed by the romans. Gaul probably emerges as the most powerful state, followed by Dacia and Macedonia. Gaulish population is still largely gaulish speaking, whith latin losing influences, it might stays a important language to communicate with other culturues or other barbarians who otherwise cant understand you, but there is no church which would allow latin to become a language which can have a long lasting effect, only nobles would learn it. Southern gaul slowly gets celtized by their gaulish elite. the majority of the central cities and large cities are in northern gaul, maybe around the region of modern paris, IIRC that was already the main capital of the gaulish people, i just forgot what it was called.
Dacia probably comprises of whole modern Romania, parts of bulgaria and Hungary till the danube. western of danube we have the baltic kingdom, the border of the baltic kingdom is roughly around modern vienna. In modern austria, in the alpine regions we have the Noricans, they are speaking a language similar to gaulish, however are rather sparesly populated, mainly because of the mountains. the noricans are slowly getting replaced by germanic people in the north, there will be some conflicts.
With no big lasting and influencing empire, all the realms are kinda divided and different, they all folow their customs and religions. However spreading of new religions is also easier without rome which would kill christians. so christianity has chances of spreading even in this timeline, albeit being different, with many different churhcs, so no roman catholic or orthodox one, but there will be a gaulish church, a italian church, a macedonia church etc.
Macedonia is made up of albania, parts of illyria and anatolia. it will also be the first area in europe to be christian, when the people adopt christian religions more and more, till even the noble and kings convert. around 300 AD or so, they will adopt christianity as main relgons. Greek will be the language of the church. other countries wont get christian for longer times, the nearer greek they are sooner they get christian. I assume dacia gets christianized around 300 AD.
By 300 AD, i expect the baltic country, comprising of hungary, parts of Austria, slovakia. being roughly the size of the moravian kingdom. thanks to dacian influences, they also adaopted dacian weapons and dacian symbols such as the dragon, and baltic people themselves have a liking of snakes, ancient lithuanians kept them as pet. They start to sue their shields and banners with dragons, wolf heads or serpents, like the dacians do. they also adopt falx and similar weapons. intrested in the rich iron mines of Noricum, they start a full invasion in Noricum. the country, plagued by germanic invaders, is rather weak, they manage to siege their capital and gain control of it. however the germanic tribes, also want to invade noricum. so there wil be fights, which the balts win, because the germans were already weakend due to the fights with the norican people.
However, they are in pressure, while the dacians are more or less good allies, in the south there are the Greeks, northwards are germanic people. This pressure forces them to become rather aggressive and change their tribal lifestyles, instead they adopt a duchy like system. One king rules the people. They also adopt a successor system, after the king dies, its normally his oldest son who replaces him. by this time the balts already converted to christianity. its around 400 AD now. Gaining strenght and wealth because of the rich mines of Noricum. the balts conquer areas in modern germany and czech republic, their influence gets stronger. the dacians expand eastwards, reaching till the caucasus now, ruling the black sea. Macedonia rules the mediterrean, with Anatolia, southern italy being under their rule.
Gaul expanded over Iberia.
also this is not my final plan, just a idea what could happend, and if migrating to france is not possible, then it must be conquered later somehow.