StackExchange-like Q&A engine
Stack Exchange pioneered a new category of website that has proven its usefulness beyond any doubt.
There is no reason to compete directly, say, with Stack Overflow. But imagine if there was no Wordpress, and Medium was the only way to blog. Not cool.
The world needs a top-quality open source alternative that works as well as (and, ideally, has feature parity with) the proprietary leader.