So really just redirect ships and settle. BUT the question then becomes; why would Rome immigrate to East Africa when Arabia gets what they want with trade and minor mixing on islands and along the coast line?
Why would Romans move to a place of little value to them? Also how could they deal with "macrobians", Waq-waq raiders, diseases and Indians who are conquering the lands further south themselves?
It wouldn't last, a romanization would fail to last as the horticulturalists adopting South and Southeast Asian crops sweep the prior people out. With sustained contact and fortification you could have a romanized Swahili Coast that is subsumed by Persians as OTL.
Its so extreme compared to just trading with native peoples and letting them be and more profitable too.
Well, some Romans have been known to have been... forcibly resettled, but as I said before - it isn't that the location and its resources are in themselves valuable, but the locations. Constantinople isn't known for having gold deposits, but for being at the mouth of the Bosporus. That is its value - and the value of any settlements are being able to trade, and control the trade. Just like in later periods, paying locals to hunt Ivory for export is lucrative.
Indian conquests of East Africa? That is a new one for me (and I had to look up Waq-Waq, do you have a resource for that - all I got is references to the Zanj, who the Chinese and Indians weren't impressed by culturally, apparently).
In addition, why wouldn't the Romans (and any local allies) also adopt south and south east asian crops if they prove to be superior? Plus, who is to say that Persian domination is guaranteed? A Roman Empire that has that much bullion being brought back - and assuming that controlling the sea lanes is profitable (it would have to be), then they're in a better position to deal with any Persian influence - in fact, East Africa might be an important minor front in any Romano-Persian war. After all East Africa is still very far away for Persia too.
If you're objection is to the whole "Romans appear and boom magically everything is better", I concur. But Roman military trading settlements, with allies in the local communities is very different to settler colonies - I imagine that it'd more resemble a network of allied and unallied tribes (like pre-Caesar Gaul).
@Nabongo Mumia II - You've got to understand the sheer distance that East Africa is from Rome Proper - they wouldn't be able to deploy large forces. Smaller forces with fortifications, and allies is the only real way to make a Roman East Africa work, at least until those allies have become "Romans" themselves. I don't see any settlements become metropolises any time soon. Small trade cities, and maybe one major city under Roman control, but the vast majority - Romanized local communties that have also adopted crops from S & SE Asia. Between this 'Exarchate' and Rome are dozens of client states, or other groups.
Sailing down the Blue Nile, is however, a death trap. The Sudd is not to be messed with, and the Blue Nile is hard as heck for Roman ships to navigate.