'Start adding things like drivable cars
So, first, Starfield is Creation Engine and does have driveable vehicles. I think that if that's the concern, they could do that via releasing the Fallout content on a current version of the engine. Which...frankly, I would like.
But, secondly...
Honestly, I don't really feel like that'd actually add all that much to the experience. Not that it'd be bad, but I think that the game doesn't really suffer much from lack of them. And it'd have, well, balancing effects. Like, a deathclaw is pretty scary if you're someone on foot. If you can just drive away faster than they can run (or, like, crash into them with a car...), it kinda changes the feel of the game.
Maybe you can rebalance that, but then you've got maybe NPCs in vehicles (and the associated technical work), and melee creatures being eliminated and...just...it's necessarily a different environment than someone with a backpack and a gun and a dog on foot.
Thematically, I mean...the Fallout games mostly kinda take place in what amounts to a fallen civilization. I guess that there's The Institute in Fallout 4. But for most of the series, you kind of need a big supply chain to build and maintain automobiles. Fuel alone isn't trivial, if you think about the world-spanning industry that it is. In Fallout 1, where you could acquire a car (though not experience driving it, just use it to rapidly move from place to place) fuel was still a problem. Like, the Fallout series has generally been about someone wandering through the wreckage of what was. Vehicles are something we can use in daily life because we aren't living in a post-apocalyptic wasteland.
Another issue is map size. Like...I think that you need to have a certain density of scripted, hand-crafted events on the map to let people just stumble across interesting things at a reasonable clip. Once you introduce vehicles, you can move across a map much more quickly. So do you let someone zip across the map in short order, lose some of the feeling of scale and make things feel smaller in a vehicle, or do you reduce the density of the placed content, make the world feel empty on foot?
Like, in general, my own feeling
and I'm sure that there are people that will disagree
is that while I like Bethesda's games, if they err, it's on the side of being too broad and not "deep" enough for given functionality. So you get things that feel kind of like they added an engine feature and just enough gameplay to show it off. Like, they have an in-game building feature that's really cool, but they haven't done a whole lot with it from a gameplay standpoint
they had that one battle with the Mirelurk Queen with placeable defenses at The Castle in Fallout 4, which was probably the most-notable.
In general, I'd rather have, say, a better-balanced perk system and better use of the existing engine functionality than spanning out into more stuff, going even more-broad and more-shallow.
Starfield has a visually-impressive terrain generator. Like, Bethesda can make some quite pretty procedurally-generated terrain...but they never integrated it into gameplay. Like, the combat doesn't depend much on terrain, so there's just not a lot of point in the terrain being changed up. When you fight enemies, it doesn't matter much whether you're in a canyon or an open plain or on a hill. The reason roguelikes do well with procedurally-generated worlds is because they generate factors that affect gameplay, make you change up how you play. Bethesda spent a lot of effort making that terrain generator...but didn't really get around to making much gameplay with it.
If they start going out and adding a lot of vehicle stuff, that seems like it'd make the world even broader. I mean, I've already seen people say, in Starfield, that okay, sure you can get a vehicle and drive it around, but there's not a lot of point. It's not like they built a Mad Max-style driving-oriented game. That'd take a lot of work to do something like that.
All else held equal, sure, I'd love if they did all that extra stuff and threw it into Fallout 5 and fleshed out all their existing features. But...I just really want fuller use of the existing functionality they have, more gameplay that uses that, and doing that competes with development resources for adding new features that then need to have gameplay built around them.