Make Workitem templates first class citizens
It should allow creating workitems based on templates (which have pre-filled data). Creating hierarchy of workitems based on template(s). e.g. creating a template for Product Backlog item such that it would include the standard tasks (Sprint Backlog Tasks), Acceptance Tests in them (like Code Review, Writing Test Cases, etc). Organizing those tempales in folders. Some of this exists already, but the main missing piece is that you would have linked templates that would also be applied.
Scott Brickey commented
Not to discount some of the good suggestions here, but prefilling data is faily easy using the process template editor in TFS Power Toys. Though I don't recall offhand whether you can define "calculated" values.