My strongest wish would be to be assured of the durability of the software provided by ST.
I started with the defunct SPL, and now there are HAL and LL drivers. But the HAL is not based on LL!
We rather use the LL which allows to develop drivers and libraries better adapted to specific needs. LL drivers are not yet available for the H7 series. Will it be available someday?
My second wish would be to have better-designed peripherals. It's probably too late, but a software developer should be part of the team developing the peripheral devices.
From a hardware point of view the peripherals are sometimes very rustic: no FIFO in the UART which implies a very high interruption rate (there is FIFOs in the 8250 dating from 1984!). I know there is a FIFO in the UART series H7, but it's late and only for high end MCUs... The i2C is very complicated to manage (Compare with ATMEL AVR series!), and different in several families.
From a software point of view it is not bearable that the I2C requires an interruption of the highest priority. The same thing for the HAL timer.
Third wish, more precise information on device compatibility between different families. I know there are documents that address this topic, but what does "partial compatibility" mean without more information? I dream of a document for each peripheral that clearly indicates what the differences are and in which families they are available.
Apart from that I find the ST policy on NUCLEO and EVAL cards very useful. And the integrated and detachable ST-LINK programming probe is really a plus.
