I would love to see additional question types for assessments such as ranking tasks/ordering.  Educational Research, especially in physics and other sciences, show that ranking has been a well studied, effective technique to demonstrate student mastery of topics.

Multiple Select type questions are also being used by the CollegeBoard to measure student understanding. Being able to create those in K12 would help give my students additional practice.

I'd also love to be able to input algorithmic questions for math and science. Adding variables to questions and accepting typed algebraic answers can be found in online assessment tools such as WebAssign.