Unused signals

If you are using 16-bit memories only can the unused signals (DATA31-DATA16, BE3_, BE2_) be left unconnected or should you add a pull-up/down to these signals?

for 16 bit device use D16-D31 (HW manual page 1-15) Unused D0-D15 should be pulled up with 10k resistors

Watt will happen if I don’t have these pull up resistors.

It’s an engineering practice to put pull ups on the unused ones. You can’t leave them floating you need to terminate them meaning pull them high other wise the effects would be unpredictable.