What are your most tempting/daunting databases that you haven't got a chance to put through their paces?
And, a second question, if you step back and think about the various APIs you've had to use, have you personally developed favourite styles of API to use?
I'd love to do more work with predicates in general. That's an open research problem I've been noodling on for years. Pretty much any SQL DB would be a good candidate for that work!
I'm gonna be a weirdo and say I actually loved Fauna's FQL. A little Lisp-ish functional language for queries is a great way to interact with document-structured data. SQL is fantastic for sheer breadth, though its specification is a nightmare and actually writing portable SQL is real challenging. One of those places where a stronger spec and conformance tests would have really helped.