I agree, it's very time-consuming to make nice-looking preview images. Being able to simply upload a model for a 3D preview would save a lot of work and give the customer more information.
The ability to hack 3D previews is concerning, but I feel like overall you would gain more sales for the good preview than you would lose to hackers. You could always let sellers decide whether or not they want to offer a 3D preview.
Animated gifs would also be nice. As a buyer, I am often hesitant to click on YouTube previews. A short animated gif would be preferable.