PHP styled forums and Boorus would be cool. I know some PHP forums have recently added activitypub support, but it’s still pretty recent and doesn’t have much of a userbase.
Also a video platform like YouTube in a way. There’s still a lot of issues to solve with that though, like storage and monetization. PeerTube just isn’t going to cut it. Realistically if this were to ever happen, it’d have to be YouTube themselves to add activitypub support themselves and let people connect to them.