I just went through the Top Gear story, and man that just drives home the "what if" to me... What if the Lego expansion had been Top Gear instead?
I enjoyed the Lego expansion but I feel they went rather conservative on it. That may be due to time constraints, game engine limitations, licensing issues, or a mixture of all three, but there's no building and that's kind of the big point to Lego's. You don't get a Minifigure to dress up, you can't build your house however you like - you can't even choose where to place the upgrades you "build" from the bricks you "collect" - and you can't modify your car or create your own.
The Lego world is beautiful and well crafted, and I especially liked some of the roads and courses as they made for some good racing and high speed driving, but instead of really going for it (and making it feel like a completely new game as it was described) they mostly just rehashed the same Horizon gameplay but with Lego skins.
Enter the Top Gear story and I kept thinking, imagine if this had been the expansion. You jet off to some crazy remote location(s) - deserts, mountains, jungles - to do a bunch of crazy stuff in cars ranging from exotic hyper cars to tractors on steroids to "reasonably priced cars." That sounds like Horizon to me. They could probably have come up with more than 2 cars to add as well. It would have introduced some silly fun with a heavy dose of car-centered stuff, and some Stig humor to finish it off.