Well the point of the slave trade is that without it its no longar a game about "the triangle trade"
which kind of makes it just "nice" trade between Europe and the american colonies (mostly the Carribean). So to NOT have slavery I have to (bend history a bit and) skip including Africa all together to make the game ok. Which in my view is still very much worse.
Not having the triangle also affects gameplay quite a bit since the player have only two continents (main trade hubs) instead of three.
This thread kind of took a strange turn but its interesting nevertheless.
Some of these suggestions are good and already planned, some are not fit for a boardgame I think (but could work in a computer game!)