I think your project is pretty neat, however, I think the hardware design could've been a little better.
You're currently using RS485, which is half-duplex. You could've gone with RS485 (4-wire), so you have full duplex communication.
I'm also curious why you didn't use SPI or I2C for communications. With SPI, you won't need a microcontroller on the other board, and you can run SPI over RS485 as well.
Plus, I think you don't really need the backpane, if you make it possible for modules to be put in a row, you could make it even more expandable, while not needing a back-pane.
I also think that a RTC (Real Time Clock) would be a very neat option.
Some other features that you might include (you've got enough I/O left, for sure).
- RS485 connection (to modules outside of the boards/plc cabinet).
- Real Time Clock Calender
- SD card slot
- Maybe even some low level 5V outputs