I highly doubt that Kongo would have another leader. Not because Kongo's not relevant enough, but because he's part of the civs where the LUA and the CUA are very tied together.
Kongo's palace have 5 slots for any Great Work not because kongolese palaces were especially decorated but because, without temples, it's the only place where Kongo can store relics (except for some wonders but wonders are never sure).
Kongo's CUA is very strong (at least IMO, but quicker GA, GW, GMu and GMe, atop from tremendous artifacts and sculptures, and the palace) BECAUSE the Mvemba's LUA is so "weak" (by "weak" I mean "having a malus"). Kongo is the first asymetrical civ in the game and was the only one until Mali, Maori, Canada and now the Mayas.
But now, if you give Kongo a new leader that can found a religion, Kongo would probably become OP because it would have it's OP CUA with a regular leader (and even an UP leader would work, the Netherlands are a very good civ despite the abysmal LUA of Wilhelmina).
Rather than a new leader, having the possibility to build holy sites BUT not earning Great Prophet points (ala Maoris) would be better IMO. But a brand new leader? It doubt they will do it because I doubt it will be in itself a good idea.