Format migration seems to me to be a key feature - the ability to lift ephemeral discussions into more permanent and deliberate mediums as required. Something like chat -> forum -> wiki, for example. Have set processes in place for determining when something can usefully be pushed upwards. Also encourage a culture of pointing to documentation /first/ to ensure the more permanent documents are well-maintained and robust.

Stack Overflow got close to this, but is still quite a ways off. The chat element is present but very much cut off from the site, and it lost its way over time as its focus changed. It may be worthwhile trying to assemble recommendations for how to select your own tools that give you these benefits, and perhaps to ease migrations between them.