This might sound rediculous but I agree that trucks would be interesting. A heavy vehicle with a high centre of gravity and a suspension with a much wider range would be nice.
Though I don't think drifting them is cool.
And by trucks I mean those champion rally trucks and possibly an SUV
Racing trucks could be a hell-of-a lot fun in LFS. They goes pretty fast, and due to their heavy weight and size, I belive it would be a great challange to completely master.