What about a few African powers? The medieval Igbo Kingdom of Nri strikes me as one of the most interesting prospects for development in an ATL, with the unique nature of its government, society and religion. One of the few states to have ever arisen which can be described as a religio-polity, the Kingdom of Nri was ruled by an elected priest-king, comparable to the early Popes, known as the eze Nri. Unlike the more paganist and animistic religions which are stereotypically deemed to have been universally practised across pre-colonial Africa, the Nri believed in an omnipotent, omniscient supreme deity, whose being encompassed the entirety of creation. They believed that 'The Light', Anyanwu, was the symbol of perfection that all people should aspire to, and that Agbala, the collective spirit of all holy beings (human and non-human alike), transcending religion, culture and gender, was entrusted to lead them there. And in all of West Africa, it was the only region where slavery was explicitly forbidden; from the rule of the 10th eze Nri onwards, all slaves who set foot on Nri soil were considered free.
IOTL, the Kingdom of Nri fell into decline and its eventual demise because, in the end, it proved too idealistic to cope with the pressures and demands of the outside world. Ultimately, it had no way to protect the freedom of its people, either from infringing slave states or from the British slave traders when they came to claim their share of the Atlantic Slave Trade. The eze Nri wielded no military power, with the Kingdom of Nri maintaining its hold over its settlements and expanding further into new territories, not through military force, but by employing a class of missionary nobles to obtain them by ritual oath. They preached and practised a policy of religious pacifism, asserting that violence was an abomination which polluted the earth, and instead employed a form of excommunication (isolation from the wider community, in essence comparable to prison when declared against individuals and comparable to imposing a blockade when declared against an entire settlement). Given that the Kingdom endured for almost a milennia, the policy was clearly used to great success within their own borders, and greatly boosted trade in the region; but in the end, when faced with profiteering adversaries over whom their faith had no influence whatsoever, their no-violence policy would have inevitably led to their downfall.
So, for an ATL, you could have a Kingdom of Nri which, at a relatively early stage, abandons its policy of religious pacifism for a more militaristic 'crusader' mentality, with the eze Nri's role bearing an even greater resemblance to that of OTL's Roman Catholic Popes during the same time period. Their nobility, spreading the word to neighbouring communities where they still practise Nri taboos such as slavery, takes on more of a 'Holy Order' vibe, provided with weaponry to counter raids in more lawless regions and authorised to use force to defend their assigned areas. In later generations, when other surrounding states begin encroaching on its domain, this theocratic militarisation develops still further, with the Kingdom of Nri raising standing armies from the wider populace to combat them, pushing them back out of its Holy Lands and eventually conquering them one-by-one.
With its vastly enhanced influence extending across the Gulf of Guinea, the Atlantic Slave Trade in the region is profoundly affected, effectively nipped in the bud when the Nri Empire repels the first European slave traders who come to the region looking for human cargo- and even outside its dominion, where the slave trade still continues to grow inexorably, the Nri religion, 'Odinani', and by extension its religio-polity, proliferates just as quickly among slave populations, driven by the growing desire for freedom and equality increasingly denied to them by their slave masters. With the example of the Nri religio-polity to serve as a framework and facilitate cooperation among diverse slave populations, slave uprisings both in Africa and in the New World are far larger, more organised and more widespread, and correspondingly have a far greater degree of success. With the Nri Empire finding itself at the centre of trade, diplomacy, religion and culture between these recently liberated nations, the eze Nri finds himself with the perfect opportunity to extend his sphere of influence across half the globe, and seal the Nri Empire's place as the undisputed African Great Power...