The Shelby 1000 is a Mustang with more power. It isn't a ground-up new model, which is what most of FM4's, FM6's, and FM7's DLC has been.
FH1 DLC was littered with different trim levels of existing models, or convertible versions of existing models.
I could be wrong, but I don't believe "cars Andre3200 cares about" is a criteria for Turn 10 when selecting DLC packs.
The SUV/off-roader bias has been very strange given the lack of any dirt tracks in game, but it could have something to do with FH3's popularity. Both games share assets, and perhaps this is to get a head-start of sorts on content for FH4. I'm not saying that's ideal — it obviously rubs some players the wrong way — but from a business perspective, it makes sense.
New car additions come down to a handful of variables:
- Turn 10 wants them in. It's able to source an appropriate model.
- The manufacturers want them in. They're able to source an appropriate model.
GT offers an interesting contrast. It's taking the FM5 approach: the majority of the cars added so far have been ones we've seen in GT6. The only difference is PD isn't charging for them. But out of 50 new additions, 33 have been revisits. I'm not counting the F1500T-A, though really, it's barely different from the Lotus it's clearly based on. I'm also not counting the '17 Nismo GT-R, nor the new GT3 RC F.
GT Sport has had a new category added in the latest update, while most of the other cars added have fallen into road car categories or the Gr.X catch-all. It's covered classics from the '60s and '70s through to modern cars. FM7 has added pre-war cars, a modern sporty sedan (the Alfa), old F1 cars from Maserati and Porsche, a brand new hot hatch (Veloster)...
I think there's plenty of variation. The issue you seem to be having is that it's not the type of variation you want.