The 'replacement' of Reddit may not look in appearance much like Reddit at all. Communities can center around anything. Just the other day I found a 'proudly anti-fascist' linux distribution with a community of ideologically-aligned friends all having a community around a linux distribution. Others I know have found deep friendships after initially meeting over music groups and going to concerts together. Rather than one mega-forum with many smaller forums, you may find interest-based communities all over the place!