The problem with the slave trade is that the British were against it and even though Brazil in OTL wanted to keep the trade alive, Britain threatened to blockade Brazil's ports in 1852 to supress it.
Slavery was one of the reasons that Brazil was and is one of the reasons that Brazil does have the economic woes it does. Slavery created a large plantation economy dominated that had very little industry. Power and wealth rested in the hands of a small minority.
When slavery was abolished is when Brazil's economy actually began growing. After 1889 hundereds of thousands of Italians came to Brazil to work on the coffee plantations of São Paulo state. Because of the coffee boom, other immigrants poured into the country, and Brazil experienced the largest net migration in history after the United States.
With slavery you wouldn't have the need to industrialise. The wealth and power would remain in the Northeast. Slavery tends to make elites lazy, and the more an economy is dominated by slaves the poorer it tends to be in the post-slavery period. Just look at the U.S., Mississippi, Alabama, Louisiana, the states where slavery dominated were the least innovative and industrialised in the post-1865 period. A cheap supply of labour is good, but you need economic mobility and meritocracy to really achieve prosperity. Or else you just have a relatively stagnant economy with a large underclass.