Not exactly adding onto the bus, but here's an idea;
Buses that look like they were once school buses but were turned into Evac buses and abandoned when everything got out of control, and you'll find some equipment on them, like medkits or pills. They'd be found in places like highways/interstates, or near places like schools (of course), or near "zones" that are places that block off zombies with barricades and all, but accessable by the player, or they could be in completely random places. But, they would be rare to find.
Then, there'll be a working evac bus that will only take you to parts like different cities or to the other side of the city.
Then, there would be a normal school bus, but when you get on you'll find backpack-like items that increase your inventory space, they'd be found randomly like evac buses, or mostly around schools/residential areas.
Next, there's the church bus. It would look like the school bus but blue, and at the same level of rare as the Evac bus, and it would either have things like notes telling the player the dangerous areas, or notes with prayers on them, notes telling the player where the church is (it would be like zones but without as much equipment as them).
It would look something like this;
Next and final, the city bus. It would pretty much look normal like a civilian city bus, but inside you'll find things like backpacks (like the school bus) or small-arms weapons (pistols basically)
There would be a couple working city buses that will take you to random parts of the cities.
