Some quick ideas:
- Add basic text functions, like RIGHT(), LEFT(), MIDDLE(), TEXT(), etc.
- Support more operators, like 'contains', &, etc.
- (Potentially) allow a default value to be set by a formula, and then allow the user to edit the field from there.
- (Advanced) -- allow 'testing' of a formula in the builder by processing the formula for the first row of the model and displaying the result (or even better, walking through the formula process steps) on the builder side.