If you change OR gate (near 1) to NAND You will not need extra NOT gate (+ must change And to NAND {second connection to OR for correct logic}) always one less gate ^^
Interesting use of tri-state buffers. Good if you're going for absolute optimization of gate numbers. Actually, in the context of the rest of the circuit you could possibly take it even further, by using active-low inputs for the ripple signals. and maybe even removing the inverters on the display ROM.