Greeks and Romans traded with the coastal cities of "Azania" (East Africa) in OTL. The thing is, they usually just traded with merchants who came to Egypt, and didn't really venture into the trading network for themselves. The best way to change this is to have Augustus succeed in his ambitions to essentially found mercantile colonies along the Arabian coast. If those ventures can be stabilised, and more Romans are directly present as far south as Yemen, the chances for a more active role in the Indian Ocean trade vastly increase. If this is successful, Alexandria could rapidly become the wealthiest city in the Empire. The OTL divide between the urbane, wealthy East and the relatively poor West is going to be even greater. You could easily end up with an empire that moves its focus ever more to the East. (Maybe even a fall of the West, with the East just cutting its losses since "who gives a damn, Alexandria alone is literally three times richer than the entire Western Empire!") And ERE centred on Alexandria, deriving its wealth from the Indian Ocean, and uninterested in what's happening in the West, could easily end up actively strengthening its position in East Africa.
A more mercantile kind of empire could come to be, interested in being the great thalassocracy of the region. With that set-up, Roman Madagascar becomes possible.
. Though if settlers from Southeast Asian islands can reach Madagascar before Africans do, who knows how differently the history of Madagascar could go.
The first people of Madagascar were already mixed.
I was already aware that the Malagasy people are of mixed Afro-Asian heritage, though for some reason the first settlers on the island came from present-day Indonesia, while East Africans migrated a bit later.
The first people that settled the island were already mixed. They were not "pure" Indonesian.
Maybe a small fleet carrying the colonists for one of these trade cities is hit by a storm, Jupiter takes pity on them and they mostly survive but end up close to Madagascar, where they make landfall and decide to build a settlement. They are incredibly lucky (bordering ASB intervention) and manage to survive well enough.snip
Maybe a small fleet carrying the colonists for one of these trade cities is hit by a storm, Jupiter takes pity on them and they mostly survive but end up close to Madagascar, where they make landfall and decide to build a settlement. They are incredibly lucky (bordering ASB intervention) and manage to survive well enough.
Years later when they re-establish contact with thr Empire (through one of those trade cities maybe?) They are a small thriving community. IIRC Madagascar has big reserves of Saphires and considerable reserves of gold, so maybe they find either or both and use that as an incentive to keep commerce going and attract new people (there is always plenty of crazy people ready to go to weird places).
You are saying they were already mixing with Africans since the beginning?
Yeah, you have several problems you need to resolve here.
1) You need to have the Romans aware of it.
For that you need
2) A Reason for the Romans to physically BE in East Africa
AND
3) The shipcraft for logistically supporting anything like this
For THAT you need
4) A time period the Romans could do this in, and a circumstance.
There are a lot of hurdles for this. Sadly I think you need to find a resolution for those first.
Like, lets just handwave a conquest of Arabia Felix/Yemen, just to simplify things. Not an easy task, but neither is it impossible. You then need to go south past Somalia - and a secure port would be worthwhile there because you could strengthen your control over the incense trade further. Yay for Rome!
But then still - why go south? Gold? Maybe. Is there gold in Madagascar? Don't think so. Naval base? Bit much. Maybe a small city in the north, but going beyond that screams problems. Allies as buffer states allowing access to the gold mines of southeastern Africa? That sounds far more Romes speed.
So yeah, at best a small chain of settlements to connect conquered gold mines in SE Africa to the Roman Empire via Madagascar, Zanzibar, Somalia and Egypt.
There however MUCH easier ways to get gold, so you'd really have to explain why this happened.