I think there is only a need for a new camera and a new core module (where only the SoC, RAM, memory and modem is updated*). The screen, cover, battery, top and bottom modules can be re-used in the “next generation Fairphone”.
*I think it’s a good idea to make SoC, RAM, memory and modem replaceable in the core module. So the “next generation Fairphone” can get support for new standards (like 5G) just by replacing the modem or when the support for a specific SoC is dropped by its manufacturer, you can replace it by another.