There's a good chance it'll be something novel we can't even imagine right now. Remember, as static as the social media world seems, every few years seems to bring at least one major service that comes out of nowhere. See Instagram, Snapchat, and arguably messaging apps like WhatsApp, Slack and Discord.
That saud, I'd guessing that any successful social media app then would be:
1. Mostly private, because the over arching fears of survelliance and spying and a lack of privacy do seem to be getting to people, as does the media's tendency to take random jokes/comments and blow them out of proportion.
I think that's why Discord has caught on so much with young people now. Because stuff like Reddit being public meant their communities ended up coming under fire for every little controversy, and they constantly had teachers/employers/media folk looking there for things that could get them in trouble/cause drama. So invite only and private is likely a big part of any future platform's setup.
2. Probably more image/video based than text based. We're seeing a move away from text in general nowadays with stuff like YouTube and Twitch being mainstream, and the likes of Instagram/Snapchat/Pinterest just make that even clearer.
But it's hard to tell really. Could honestly be anything.