As a globally recognized monarch, Black Panther has a certain degree of power and prestige that many of his fellow heroes lack. This has served the hero well over the years, but it's also raised the question of what would happen if he tried to expand that power. One such reality exists in the Marvel multiverse, where T'Challa rules alongside his wife, Storm.
In the "Black to the Future" reality, Black Panther and Storm remain married and essentially rule the entire planet. Unfortunately, the path to peace was violent.
Black to the Future
The two "Black to the Future" stories were featured in two separate Black Panther annuals -- the first by Reginald Hudlin, Larry Stroman, Ken Lashley, Roland Paris, Carlos Cuevas, Jon Sibal, Mill Milla,and Val Staples, with the sequel by Hudlin, Lashley and Milla.
These stories introduce Earth-8441. In this world, Wakandan society has become a global dynasty. Storm and Black Panther never split and their marriage allowed them to weather a series of massive challenges that came to the world. In this reality, Wakanda had taken an active part in combatting the slave trade when it came to Africa. Using agents, they were able to infiltrate the system and rescue slaves, bringing them back to Africa. Many of these escapees were taught in the ways of combat and espionage and helped attack slavery more overtly.
This also meant that Wakanda was eventually exposed to the world as a genuine power far earlier and was forced to contend with aggressive governments as a result. Eventually, a tense peace was reached when the then-Black Panther issued a warning to each and every world ruler that any future conflict would be met in kind.
Pax Americana
The United States became more expansionist and aggressive. Under the command of Iron Man, the country became Pax Americana and began invading other nations they perceived as genuine threats. This included Latveria and Atlantis, both of which were largely wiped out. However, Doctor Doom and Namor both escaped with their lives despite the belief of the Americans. Doom later battled against T'Challa, who was forced to kill him to end his threat to Wakanda.
Meanwhile, T'Challa and Ororo had many children together: T'Chaka, T'Wari, Chaka, an unnamed son and two unnamed twin girls. Many of them developed mutant powers. Storm became a leading voice for mutants around the world and with Wakandan resources, she was able to expand the dream of Charles Xavier and facilitate the construction of Xavier Institutes in every nation on the planet. When Magneto tried to forcibly take Storm and her children into his army, Black Panther was forced to kill the Master of Magnetism.
Eventually, Pax Americana turned its attention to Wakanda. Talks between Tony and T'Challa fell apart and Iron Man led a massive army of Iron Man suits to attack Wakanda. Tony took part in the battle himself, commanding his forces from inside a mech-sized Iron Man.
However, Black Panther responded in kind -- revealing a massive, mech-sized panther. Wakanda won the battle, but Tony Stark was accidentally killed in the fight. T'Challa retired from his role as Black Panther after declaring peace between the US and Wakanda, allowing the two world powers to co-exist.
Shuri was made the new Black Panther in T'Challa's stead, becoming a dedicated protector of the world and hero in her own right. Decades later, Wakanda was one of the most powerful forces in the entire world. T'Challa was said to essentially rule the world from his position, an assertion he didn't deny.
It was a largely peaceful time, with his grown children now starting families of their own: T'Chaka married Danielle Cage (daughter of the current US President, Luke Cage) while T'Wari married eventually fell in love with and married Dorma (daughter of Namor). Meanwhile, Chaka had her own daughter, Grace -- although her significant other hasn't been revealed. A world ruled by Black Panther and Storm, while forced through violent times, found peace thanks to their steady hand.
While problems still exist (such as T'Challa mentioning there is still some hatred for mutants across the globe), it is shown to be a fairly kind version of the Marvel Universe -- suggesting Black Panther taking on a larger leadership position in society could benefit the entire world.