There’s been 5,000 years of history of them as separate kingdoms and each has built a separate identity and likely has its own heroes and stories worth telling.
Therefore, additional films that could tell us more of these stories could certainly make sense.
However, with seven different kingdoms to deal with, and likely a runtime of somewhere in the two-hour range, it seems unlikely that Aquaman will delve too deeply into any one of them or introduce a significant number of characters from them.