In recent announcements, Microsoft has made it very clear that the Dynamics 365 for Finance and Operations, Enterprise edition application cannot be customized in the same way as it used to be done in the past. Currently, there are two approaches available for customizing Operations: overlayering and extensions. Overlayering allows the customers and partners to change the code that Microsoft has shipped as part of the core application. Extension is a new development model where the developer can utilize events and hooks within the application code to include additional functionalities without impacting the Microsoft code. As is obvious, overlayering is easy and provides great flexibility but comes at a huge cost at the time of upgrade and also prevents customers from utilizing the continuous updates and innovation that Microsoft is bringing to the product. With the third platform update, Microsoft has already hard-sealed the platform and foundation models, which means that you cannot overlay those models. The application model, which is the last model, is going to be hard-sealed after the Spring 2018 release. What this means from a solution design perspective is that unlike earlier versions, there will be a scenario when a requirement cannot be customized. To make sure that your custom solution is feasible, consider the following points:

• There could be a customization requirement, which may not be possible to be developed through extensions.
• If there are no options available in the product to extend the requested functionality, find a workaround or take Microsoft feedback. Ultimately, if it's not feasible, say no to the customer. 
• If you think your requirement is common, suggest the same to Microsoft using the idea portal; it might become a part of the core product in future releases.
• Also check the Microsoft roadmap to see upcoming features and if those can fill the gap.