The UN, and new technology for the MCU
The mid-credits scene shows T'Challa pulling his version of Tony Stark's "I am Iron Man" just in time for the 10th anniversary of the original MCU vehicle. In front of the United Nations, the King of Wakanda pledges that his nation will stop being isolationist and help with the world's problems. When someone asks how a nation of poor farmers can help, T'Challa smiles and the scene ends before he can fully crib Tony's line.
Not only is this the logical end of the plotline of the movie, and a vague reference to how the Marvel Cinematic Universe might continue on without Robert Downey Jr.'s Iron Man, it's an explanation for how so much action in Avengers: Infinity War takes place in Wakanda, a place that remained shrouded in secrecy up until Black Panther's mid-credits sequence. In the Avengers: Infinity War trailer, the money shots that don't show Thanos and his hunt for the Infinity Stones show Spider-Man on a mechanical ring and a giant showdown on the fields of Wakanda.
During the promotion set visits for Black Panther, producer Nate Moore said that T'Challa's little sister Shuri (Letitia Wright) is smarter than Tony Stark, making her the smartest human in the Marvel Cinematic Universe. Wakanda's doors are open for humanitarian purposes, and what's more important to helping humanity than helping the Avengers stop a space invasion by multi-armed aliens? It looks like Captain America -- with some sweet vibranium shield gauntlets -- leads a charge of the Wakandan soldiers whose blankets make force fields, and Bucky is seen cocking a gun alongside the Dora Milaje. If Wakanda was still masquerading as a nation of poor farmers, this showdown probably would have happened somewhere else.