Give an additional twist to the story line by adding a new scenario with changed environmental conditions.
Delete scenarios as soon as they become redundant or out of context.
Change scenarios incrementally as some new elements have come into play.
Adding input variables or interim variables is just a matter of hours. Output variables too can be added in a jiffy!
Delete a complete set of variables and bring in a new set, if the need be. Create more complexity with nil coding!
Change variables on the fly and create new linkages. Also create more interim variables and sub-variables, if required.
A possible solution that lands in between Off-the-shelf and Custom-built solutions is the Tailor-made solution. Tailor-made solution utilizes a core quantitative model. This solution requires fewer changes to the Off-the-shelf solution. The good thing about Tailor-made solutions is that you will often be able to get almost a Custom-built solution for a cost slightly above an Off-the-shelf solution.