All the good GP dressage horses I have ever seen have had incredibly strong loins. They may be very different in other parts of their anatomy but they are all the same behind the saddle. IMHO it’s the most important thing to look for in a potential GP horse.
I bred a foal last year who had to strongest loin connection I’ve ever seen. He’s been sold to a talented young rider so I hope they go on to do great things together.
Just going to work out how to link a picture.