As a supplement to the workflow engine, I would really like to see a state transition engine for picklist values. I would like to have values in the picklist move in one direction (e.g. Created -> Submitted for Approval -> Approved -> Closed) and not the other.
Right now, I'm using a kludge where I've used a combination of checkboxes and dependency between fields to pseudo-implement the transitions I want. However, this is nowhere near as powerful or comprehensive as having a state transition engine that would automatically control flow.