If someone creates a new idea and it takes off they either get huge offers from existing companies to buy it or those companies use their huge resources to build a bigger and better version.
In the age of the start of facebook and reddit there were no mega corps taking interest or possessing the talent in these areas.
But for real, the idea of Reddit seems perfect to me its just way to popular now. Maybe a platform where you need to pass some sort of quiz created by the mods of the different communities in order to post/comment? I could see this having lots of issues though.
Also I don't really mind reddit going to shit ever since I found HN a few years ago.
FB groups have had this ability for a while. I'm in one (a meme page for a semi-popular TV show) and to keep the quality of conversation high, new members have to answer 3 questions about stuff from the show correctly to join.