What would be the most probable territory of this '' hyper-southern Africa '' could expand from that of the south to the Congo and perhaps Kenya to Ethiopia?
You could always have an alt-Scramble for Africa triggered precisely because no European power wants this nation to expand that far, in particular if it's owned by another European nation and not independent yet. If this is Portugal, we can pretty much assume that if the country stays united, there's a constant struggle between South Africa and Brazil over who
really runs the Portuguese Empire, since Portugal (or the Algarves for that matter) will have minimal pull and might rather just secede from the whole thing. The logical point to me is to stop with Angola, Zambia (possibly including Katanga), and northern Mozambique. This might affect other "Portuguese" interests like those in Guinea, Dahomey, the Casamance, São Tomé and Príncipe, and any other Portuguese African ideas you can think of, since "Portugal" is monopolising Africa for themselves.
If Brazil and "South Africa" (call it Azania, maybe, since that term has quite an ancient usage) keep united, in the sense of a personal union under the Portuguese king (who might as well live in Rio de Janeiro or *Cape Town--I like the Portuguese name of Cape Town being
"Boa Esperança" BTW), they could maybe conquer whoever is ruling in the Swahili part of Africa (Oman, Kilwa?) and claim up to the Ethiopian border. I don't see them gaining the Congo--yes, there is the Portuguese link via the Kingdom of Kongo's historic relation, but European powers probably wouldn't want Portugal to monopolise the place, so they'd grab the Congo. Katanga's fate is different than most of the Congo, since it could easily be grabbed by Portugal as their final bit of expansion, making sure there isn't that awful abomination of borders that is the OTL Congo Pedicle.
South Africa/Azania (Azânia in Portuguese?--I think there could be alternate spellings based on what I know of Portuguese orthography as well how Azanian Portuguese evolves) by themselves as a Brazil-like monarchy (or republic) could probably still do a lot of what I mentioned above, but I think they could easily be hampered by the same factors Brazil themselves faced OTL. Still, that's Angola, Mozambique, Zambia (because even the Zambezi can't be a final barrier when there's no one to stop them), plus bits of Katanga for lands north of the Zambezi River they could seize.
What these territories in interior places like modern Katanga or Zambia would look like? Maybe like how Brazil colonised interior Paraná and São Paulo states rather late? Or for that matter, Europeans in Rhodesia, both north and south Rhodesia. Definitely would be a significant amount of settlers, both European (or Indian/Chinese) immigrants and natives from the South, colonising the place. One major issue is that if there's decolonisation in this world, the later territorial acquisitions of South Africa/Azania could be affected. There's not enough time to assimilate the locals, as there would've been in the other parts of the country. There would likely be serious issues to be fought against by the locals. There could easily be a bloody struggle against the forces of decolonisation. In any case, the separatist states could at some point end up puppet states of South Africa/Azania based on economic domination.
Such a nation would be a Greater Indonesia. Maybe if the Dutch were able to colonize all of Australia as well as Indonesia, come independence Indonesia will include the dry continent down under. It would be a mix of white and Indonesian settlers, with the Indonesians probably dominating most of the north. South East Asia would be harder, since it was more divided between different colonizers and native states. But it's possible a similar relationship could have formed. Maybe instead of India, Britain colonizes all of SEA (including Indonesia) as well as Australia, and they achieve independence as a single nation.
There's 100 million Javanese alone, versus 24.5 million Australians (of all races). That's not counting the other Indonesian peoples. I think that nation would look most like OTL South Africa--extremely multiethnic and diverse.