There are quite few scenarios that easily could be made more challenging by allowing a free car choice. Of course, to avoid unfair bragging the car and configuration used should be displayed on the result screen. The idea came to me when I used a basically driving only scenario on a mod map. It it had been time trial... one could choose any vehicle but so you are limited to what the map author selected. Freeroam is not a solution either, many mod maps are so large and complex that it is easy to get lost and it would take enormous effort to get the precise same route. You actually can spawn a different vehicle into scenarios... however they dont trigger checkpoints. Thank you for consideration.
As long as the author has the control to restrict vehicle selection then we shouldn't run into the issue of cheating/breaking the scenario.