I know that DNG is based on the TIF file format, and that in a DNG file the main IFD typically contains a low resolution preview image, while a sub-IFD contains the actual full resolution raw image. I'm writing my own software that will generate test images in DNG format, and I was wondering if for the sake of keeping things as simple as possible, if it would be possible to make a valid DNG file with no preview image, and no sub-IFD, and instead store the raw image in the main IFD? Would such a configuration be a valid DNG file? Or would that violate the DNG specification?
It's not recommended. If you're going into the teritory of "not recommended", you would need to look at exactly which DNG readers you need your images to be read by. Some will only handle fully compliant DNG files.
I don't know what your custom software will do, but I'd suggest having that preview (which doesn't at all have to be low resolution JPEG). There's lots of advantages to this data if you want to go there.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"