Going by the OP, I'd say Madagascar would be a strong contender. Perhaps united under the Sakalava Kingdom instead of the Merina Kingdom? With the Sakalava's long history of garnering their wealth through naval trade networks in the Indian Ocean, if they'd got on board with the European traders early on, during their first few voyages around the Cape to India, the Malagasy Sakalava Kingdom could have united the island far earlier than IOTL. They'd be ideally placed to dominate the trade routes between Europe and Asia, located slap-bang in the middle of the trade routes- essentially, they'd be taking over from the Ottomans as the primary trade intermediaries between Europe and Asia immediately after the discovery of the sea route around Africa breaks the Ottoman monopoly over the Silk Road, and they'd probably hold that position for around a century or so, until technology advances to the stage where the trade can become profitable enough for private European enterprises (the East India Companies) to take center stage- long enough to make it plausible for them to attain similar levels of wealth to the contemporary Ottomans by the start of the 17th century.
From there, if the Sakalava merchants have cottoned on and switched the focus of their naval trade missions from supplying the Arabs with slaves to the far more profitable business of supplying the Europeans with Indian spices and African gold early enough, they might even be able to out-compete a few of the European East India Companies, especially if they can maintain their early dominance of the East African gold trade by striking out across the Mozambique Channel and imposing their own rule over the territories south of the Zambezi before the Europeans do. It's also worth mentioning that the native Madagascans did originally come from Indonesia (South Borneo, to be precise), so if TTL's Madagascar did somehow manage to gain control over some of the islands in the Indonesian archipelago, they'd have a much better chance of maintaining control of those islands into the post-colonial era than the Dutch ever did.
And Madagascar's one of those few places in Africa where those mineral resources that you're going to need in order to establish an industrial complex are all there in abundance. Coal, steel, natural gas, oil (along with titanium, chromium, cobalt, copper and nickel, all of which will be extremely useful when it comes to producing more advanced alloys later on); on all counts, the reserves on the island of Madagascar alone exceed those of the British Isles prior to the Industrial Revolution. So, here you could potentially have a single African nation ITTL which could easily encompass Madagascar, Mozambique, Zimbabwe, Zambia and most of Indonesia- perhaps even more, if they manage to expand further across Southern and Eastern Africa before the Scramble for Africa starts to build up steam. Realistically, such a nation, this 'Empire of Madagascar', would have a population on a par with that of OTL's USA, and a GDP (PPP) per capita similar to that of OTL's Turkey, giving it an economy larger than that of OTL's Japan- perhaps even reaching a par with that of OTL's India. If it had a similar military budget (% of GDP) to Turkey as well (which is marginally lower than the world average, might I add), this nation would also have the third highest military expenditure in our world; higher than that of Russia, and exceeded only by the USA and the PRC. Bearing all of these things in mind, would TTL's Madagascar be touted as one of the world's superpowers ITTL? Maybe...