I'd like to see the ability to declare runtime calculated entity attributes to simplify app design and improve application robustness and consistency. The attributes would not be created or stored in the database but would be available in webscreens, actions etc like a 'normal' attribute. It would be good it they could be simple calculations based on entity fields from the same entity, aggregates from related child entities or based on result of an action.
e.g. the classic Invoice and LineItems
Each line item may have a caclutated attribute "LineTotal" which is a simple LineQty * LinePrice calculation
The Invoice would have an InvoiceTotal which is the the Sum Of "LineQty * LinePrice" from the related LineItems entity
There may be more complicated fields such as tax calculations that are calculated in an action. e.g. for a combination of state and federal taxes dependant on which state the invoice customer is from.