I use the word functional instead of usable or controllable because that’s kind of under the umbrella of how they could be implemented.
Both Fallout 3 and Fallout 4 had rail systems you could see moving (the presidential metro and Nuka-Express) opposed to Fallout New Vegas (Camp McCarran’s monorail) for example.
I feel like the two major concerns would be how their addition would effect map design and how the engine is going to handle them.
They should have live mounts of some sort. There are plenty of rad stags and giant rad scorpions in the world, so there is no reason we shouldn’t think mutated horses or other types of trainable animals exist.
They could also do some sort of motorcycle or even something like a sentinel bot you could ride on.