In this scenario, it would be best for Mayan to build a list of functionality requirements and use it as a checklist during testing. This approach allows her to clearly define what the new features should accomplish while ensuring that existing functionalities are still working as intended. It also helps in identifying any impact the new features may have on the existing ones, facilitating a more controlled and structured development process.
While considering multiple solutions is also important, having a clear checklist will provide a more systematic way to manage testing and integration of the new features.