This is a huge request, but I would love to see http://data.stackexchange.com/ built on top of the Stack Exchange Q&A engine.
The data explorer is great for exploring on your own, but really bad at searching for other people's answers or even your own past queries because everything becomes duplicated for every time the search is run, final product or otherwise.
One of the issues that can be addressed is the ability to delete your own query on data explorer.
If there's a need to keep records of searches for auditing purposes, surely this can be done on the backend.
People without a great understanding of SQL could explain what they're trying to do as a question. And other people could provide the execution by answering the question. We'd have automatic version control for people to update their own answers and a mechanism for other people to provide different answers or different questions structures. Voting on answers could help get good answers to the top.
If the query itself was built with a tool like Stack Snippets, each post could have a syntax highlighted version of the code and the ability to run inline. If you wanted to edit the query, you could open up the post in a full screen editor similar to current query page.
People who just wanted to write an ad hoc query could post a question and answer at the same time. By adding room for free text in the post description, past queries could be much more easily searched and explained.
An integrated service could provide better linking between accounts. In the future, perhaps that could be opened up to queries against things that require authentication. 10K users could query deleted posts. Individual users could query their upvote and downvote history on a particular site using that sites.
It's a big request, but could provide a much more consistent experience and branding across different Stack Exchange tools.