I think you should not look at it from the moral point of view. But from the "what player is going to do/player's morality" point of view.

For example, in case of a game when you run a sugar plantation and you (the player) is personally going to whip your slaves, well, it's kind of outrageous :) But when you have a game like Victoria 1/2 where you have those slaves as an abstract category of population and the player is not catching those slaves and not doing anything bad to them and even have an option to free them, well, it's completelly OK.

It's nice to see so many moral people, but let us not forget about the players who install Fallout and Skyrim mods just for the ability to kill the children NPCs along with the adults. In games, players don't always make the moral choice, and that's why we have the thief class. From a game stance, slave trade isn't too different. Both are usually deemed immoral and can get the character in trouble, as well as a risk-reward system. Just like a thief has a chance to get caught pick pocketing, a slave trader in the game may have a chance of a slave mutiny on his or her ship. It doesn't have to be glamorized, or a major focus. A play can buy and sell trades in certain ports, but may not be allowed to in others (as was the case some times). If a mutiny happens, the player suffers a loss of something, perhaps valuables. If you're still worried about the controversy of a historical economy, try making it an option for the game to be played without the slaves (I've seen many game manuals tell of other ways to play the game, this won't be much different).

It's nice to see so many moral people, but let us not forget about the players who install Fallout and Skyrim mods just for the ability to kill the children NPCs along with the adults. In games, players don't always make the moral choice, and that's why we have the thief class. From a game stance, slave trade isn't too different. Both are usually deemed immoral and can get the character in trouble, as well as a risk-reward system. Just like a thief has a chance to get caught pick pocketing, a slave trader in the game may have a chance of a slave mutiny on his or her ship. It doesn't have to be glamorized, or a major focus. A play can buy and sell trades in certain ports, but may not be allowed to in others (as was the case some times). If a mutiny happens, the player suffers a loss of something, perhaps valuables. If you're still worried about the controversy of a historical economy, try making it an option for the game to be played without the slaves (I've seen many game manuals tell of other ways to play the game, this won't be much different).

I think it is a deep mistake for players and designers to expect a 1:1 correspondence between gameplay mechanics and real life. In all our time gaming (tabletop / board game or electronic) how many innocent lives have we players all taken collectively? How much have we stolen? How many have we wronged?

If we are talking about fictional entities the answer can only be zero. These things do not exist. They are not real. Therefore no living thing can be harmed, and what kind of creatures are we if we are obsessed with the morality of deeds against non-living, non-existent things? At best we can be accused only of committing crimes of the imagination.

I agree with the point others have made that some will see your work as taking a position, and I feel we currently live in a time where being seen to be moral is perhaps more important than actually BEING moral. If position is a concern, I'd simply make it clear in the manual that this was historical fact and its inclusion is neither meant to condone nor glorify and leave it at that. Some people will nonetheless be deeply offended. It is their right to be so, and it is your right to ignore them. Let them create works that promote their own values. Maybe your work can serve as a springboard for a game about abolition or a deeper simulator whose mechanics plumb the depths of the psychology of objectification.

As an aside let me also note that a rising interest in injecting modern morality into gaming isn't in and of itself a bad thing. There are wide open frontiers to explore in that direction. But a game is not somehow immoral because it incorporates distasteful and even downright evil subject matter. With gaming we must resist the mechanism in our brain that correlates thinking about something as doing it. Thinking is thinking. Doing is doing.

Thinking is thinking. Doing is doing.

it looks more like 'slave' is just another line item in a list

if you want some real historic content, you also need to include things like :

- the carribean was known as the 'white mans graveyard' (same more than a few other colonial areas) because of the death rate from tropical diseases killing like half the europeans who went to live there.

- because of the value of the economy there were more than a few wars to try to grab others territory

- Pirate/privateer activity

- political machinations to maintain the sugar monopolies by the rich families involved there

- the constant threat of slave uprisings

- trade monopolies for secondary goods maintained by the mother country

-natural disasters (including things liek an earthquake that destroyed much of a major town)

- the religion factor

- indenturing of white 'slaves'

- mixed race populations