After a cursory look at the code I think I know what would be involved in adding a different nor flash chip. However, I'm not sure how much of that code would apply to either FRAM or MRAM. Good point on the clock speeds -- looking at them, there are some memory chips that are 100+ MHz, but many are only 40 - 50 MHz.
Does FlexSPI available for pins other than the ones you've used to support the additional memory chips? If not, how difficult would be to do QSPI using a regular SPI interface? Or would it be better to stick with regular SPI for FRAM or MRAM?