Not knowing the game world or year that game world is in, i would suggest replacing your existing fleet with new 777-300ER.
However in saying that, i think you will fine that perhaps replacing them with 748s would result in an increase in fuel usage and seat costs against the 777-300ER.
But i would also suggest the 787-10, depending on the game world, to replace the 777-300ER. While not having the same seat capacity as the 777-300ER, you would see a benefit in less fuel usage and less operational costs in using a newer aircraft, but needed to increase frequency to match the current capacity offered by your existing 777-300ER fleet.
787-10 is really uneconomical. You may as well fly 9s or even 8s instead because of how little difference there is in seating capacity.