I think FP5 will be great with better compatibility with custom ROMs like CalyxOs or GrapheneOs and some security improvements.
I have read that the bootloader seems to be buggy when installing CalyxOs, with the risk of bricking the device. I hope that this kind of problem will be solved and that Fairphone will be able to add the Calyx keys as trusted to the bootloader.
I think it would be great if Fairphone supported GrapheneOs. They said something like FP4 didn’t have the basic security requirements to develop their OS in this device. I hope that can change with FP5 (Frequently Asked Questions | GrapheneOS)
I also think it would be great if Fairphone could develop a kill switch for some modules and sensors. This would increase the security and privacy of the device.