If Europe hadn't pretty much started it's presence in the Western Hemisphere with trying to establish plantations in the Caribbean, I can see the demand for imported African slave labor being a lot less, at least in the 16th Century.
So if instead of Columbus sailing west on behalf of Spain, you might see a different navigator (eg Cabot) sailing on behalf of England, and taking the northern route; then they arrive northeastern North America, maybe making it as far as the St Lawrence river in the early years, and have to make their way south to find a way "around" this "landmass", so they can arrive in Asian markets. Then, some years into this exploring, they find the southern tip of Florida, and when they sail due west from there, they find Mesoamerica.
Important thing to note in this scenario is, the caribbean islands and most other prime agricultural real estate would end up being an afterthought; by the time anyone found out that these people had gold trinkets, they would already know about the truly wealthy civilizations on the continent to the west. So there's way less pressure early on to someone find a way to make the "west indies" profitable for Europe.