On the positive side, slavery gives benefits in economy and possibly manpower. On the negative side, history condemns it, and player feels (or at least should feel) bad when thinking about slavery. What were the in period points against slavery? Think about those, then model them in the game.
Unfortunately, you would likely not see large-scale movements against slavery until the very end of the game, if we want to model the kind of liberal abolitionist tradition (and even that was not set in stone - it struggled with the notion of 'responsible government', and slavery was not abolished even with the very radical demands of the French Third Estate in the initial events of 1789, as potentially harming a source of revenue wasn't seen as a great idea in the fiscal crisis).
Beforehand, you would see the 'you can't enslave people if they are your own religion' argument, or the 'you can't enslave New World people who haven't had the chance to know they're not heretics', which still allows you to enslave other groups of people and so isn't a great argument for a modern day player that should, I hope, care about all people across the world. You could have events showing the negative side effects, but that could also be exploited by bad-intentioned players who revel in the suffering. So it's pretty tricky.
I think a lot of people are confusing "slavery was good for the kinds of people who wrote histories and have histories written about them" with "slavery was good for the national economy writ large." It depressed wages and made it easier to keep the poors under the thumb of the wealthy; do as I say without complaint or I'll replace you with a slave, but there's a good reason most of the European nations had banned slavery in their European empires long before they banned it in the colonies and why they slave empires of Iberia and the UK (among others) tried so hard to justify the slavery of *others* in religious terms.
That's probably not wrong if you're talking about the economy in a holistic sense, and assuming slaves can uniformly fill the role of any worker. However, in the kind of Triangle Trade slavery that would likely emerge during most games, that is not really the case, since as you said, it is only being carried out in the colonies, specifically for the kind of extremely intense work that cannot truly be done safely but still produces a very profitable good (like sugar) and is hard to induce people to migrate over to do that kind of labor.
The racialization of slavery also meant that, essentially, slaves and free people were hardly ever competing for the same kind of work - a white Frenchman would not migrate to Saint-Domingue to work as a domestic servant, as it was known that all those jobs would be filled by slaves. So instead of keeping the poor people under the thumb of the wealthy, you had things like the poor artisan worker petits blancs of Saint-Domingue looking down upon the wealthy free colored people who owned plantations.
So, while you could say your economy would take a hit if you start with slavery in your own metropole, it is pretty hard to justify the player seeing an immediate economic hit to themselves if a certain kind of specialized system is set up at the right time.
Edit: Though of course, a big downside to that kind of intense slavery is uprisings, which would involve a huge number of the populations and see a massive amount of attrition for anyone trying to put it down, thanks to tropical disease