Q. Including slavery and slaves as a commodity will definitely make the game more somber. After all its entertainment and meant to be fun to play with friends. Do you want to be reminded?
I think you misunderstand the cultural meaning and relevance the triangle trade. Including slavery, when the player is playing a slave fighting for freedom is very somber. Including slavery where the player is rewarded for crimes against humanity is, for some people, sickening. It's not just the nearness in time, but the depth of the atrocity. The game you propose is akin to asking "How can I make a great WWII Concentration Camp Tycoon Game?" If you have heart heart set on a WWII Concentration Camp Tycoon game, then you should be aware that you are dedicated to an offensive game. You can't change the impact of slavery or the Holocaust just because you want to explore it.
Q: There is some gameplay benefits to include it (slaves and weapons as "goods". Another continent makes travelling more complex etc). Also, it seems almost LESS respectful to the suffering to simply pretend slavery didn't happen (and especially in this setting, where it was a HUGE component).
I think you're projecting what you want into what others might consider respectful. Generally, slavery isn't brought up, even though it's financial effects are a huge part of everyday life. This is not considered disrespect. Rather, when it is brought up with insensitivity and ignorance, when it is portrayed as less horrific than the Holocaust, this is often considered disrespectful.
The general respectful way to portray slavery is to put the player or main character of your story in the role of escaping slaves. This allows you to talk about slavery, while maintaining that slavery is bad. This can be somber, but it can also create even more thrilling victories because of the immense stakes.
If one insists on making a WWII Concentration Camp Tycoon game, then the 'respectful' way to make that game would be to subtly weave this villainy into the mechanics of the game. Instead of making the game about winning, it could be about 'losing last.' Alternately, for something a bit broader, like the triangle trade, one could make the trade of slaves an implied lose state, something shameful, something that comes with a long term loss. Perhaps slave uprisings could be a big thing, and that loss be portrayed as a good thing.
But no, you can't treat this all too recent part of world history as just a simple commodity. You can't divorce the cultural legacy of the Triangle Trade from your historical analysis through this game. Same if you wanted to make a game about the current day black market and wanted to include the sex slave trade of children as part of your game for realism's sake and in the interest of 'respecting' those poor children. If you make evil things good, your game will be sickening to most empathetic people.
