I didn't realize this was an unpopular opinion until recently. but if I had to name something positive about slavery, it would be that without it my girlfriend's ancestors would never have been in the US and as such we wouldn't be together right now.
Part of why this opinion might be unpopular is because it is worded rather badly. This isn't "something positive about slavery". It is something positive about the fact that your girlfriend exists, and she would probably not have existed without slavery, but that does not mean slavery itself has any positive aspects.
Similarly, I would definitely not have been born if the Nazis had not taken power in Germany and subsequently annexed Austria, as the latter event led my grandfather to flee to the Netherlands and, later on, to meet my grandmother. This isn't something "positive" about Nazism or the Anschluß, though, and it would be in extraordinarily poor taste to formulate it as such.