Uh, I'd love someone to have a try at full-blown direct democracy. Most aspects being controlled (and ideally owned) by the very same people who use the platform. Not sure if that's good or feasible, though.

And what I always love is to see design principles that foster a nice, amicable atmosphere. Some online communities, games etc have aspects of that. It's somewhat more rare on modern social media. I sometimes wish hanging out on the internet was a bit less about politics, trolling and memes, getting agitated amongst random anonymous people. And a bit more like an evening at the Irish Pub with friends. Or getting to know new friends there.
We do things like that. I just think good platform design still has potential to achieve way more than we currently do.