Your proposal about the neutral/ground bar is certainly permissible. But functionally it improves nothing-- not worth the expense and effort. Of course to make the panel neater inside is important if viewing it is part of the New Year's Eve party entertainment.
Seriously, the unused breakers could be left in place to fill the open spaces in the panel front or else blanks specifically for your panel must be installed. Open spaces in the panel front are not permitted.

Where I reside it's required to have 240v receptacles for Ranges and Dryers even if you've got gas appliances currently installed. Might want to check your local codes.

I'll be doing the work myself. It'll only be the cost of the bar and some time. I am capable of performing the work. Have done heavy ups before and have had inspectors approve them. I just don't know all the "rules" so to speak.

Where I live there is no reqjirement to have electrical setup for range/dryer but I will be leavi ng the wires there. I am only going to be adding a single breaker to make a minor change. Just want to know the rules as I said. There will be no open spaces in the panel face. If there are I will install a blank plate.

Keep in mind, if you simply add a ground bar secured directly to the panel tub, It will only be rated for connection of ground wires and not neutrals.