What can I put over drywall on a garage firewall?
The Code does not restrict material installed on top of “Fire wall” materials. (Btw, the 5/8” type-x gypsum board is only required on the garage side. It’s not required on the “living space” side.)

Also, you can have “holes” in the gypsum board, including electrical outlets, electrical switches, a door if it’s fire rated, etc.

Also, the garage cannot have a door opening directly into a sleeping space. 

UPDATE:

Oops...actually the Code does restrict what can be installed on walls and ceilings. They can’t have a “Flame Spread Rating” greater than 200, unless it’s less than 1/28” thick and cemented to the surface and has a flame spread rating less than paper. (See ICC R302.9). Also, they can’t have a “Smoke Development Index” greater than 450. (See ICC R302.10)