Compromises in design or, at least, different design 
For instance, Fallout 1 & 2 had (if I remember correctly) a much less open-world design, with a number of discrete locations around which you could wander separated by large expanses of wasteland which you (essentially) fast-travelled across - in Fallout 2 using a vehicle, I gather.
The modern equivalent would be having large, empty areas of map across which it would make sense to drive, with more densely packed 'quest areas' which it would make more sense to explore on foot.
But while such a game might be Fallout, it certainly couldn't be in the same style as Bethsda's Fallout.
Obviously we know Bethesda haven't got drivable vehicles in the game, and I doubt they'll ever have them in any future game (but I may be proved wrong, who knows?
), but I think there are ways that mods, while obviously not being up to professional standard, could work well enough and be fun enough to be worthwhile.
There'd be two possible approaches, I think.
- A fast vehicle. This wouldn't be manoeuvrable enough to get through very dense built-up areas, but for late-game would be an alternative to fast-travel in that it could get you quickly from one area of the map to another - especially welcome if you don't much like fast-travel but have trekked across and explored the same terrain quite enough already.
- A slow vehicle with extra carry capacity. This would go at a sprint (at most) over smooth ground, and more slowly over rough. The extra carry capacity and the benefit of faster movement (without action-point cost) would be balanced by the cost of obtaining and maintaining the thing. Mostly it would be useful for packrats who just want to clear out every location they come across, but without constantly going back and forth from vault to trader to home to vault, or for people who'd like to roleplay a travelling merchant adventurer. This would probably need some armour, but only useful against weaker melee enemies. A raider with a rifle should still be dangerous, and force you to retreat or get out and fight.
Of course, a fast, armed and armoured vehicle would be fun, but that would be strictly in 'overpowered for the laughs' territory 