I believe the gaits in the front leg (and scope for jumping) can be influenced by the angle of the shoulder blade, but also by the length of the humerous, and the angle of the humerous. People don’t think about this bone a lot, because it is hidden inside the flesh. It corresponds to the upper arm in a human, from the shoulder joint back to the elbow.
The longer and more vertical the humerous at rest, the further forward the horse can move his knees.
I am around the low end of nice horses, but every once in a while I get to see the higher end. A couple of years ago I went to watch friends of friends at a schooling clinic jumping 5 feet. It’s the only time I’ve been at eye level, so to speak, up close, to horses working at that level (as opposed to being up in the bleachers). The horses definitely had shoulder configurations that were different from anything I’d seen before: the shoulder blade much more sloped back, the humerous longer and more vertical. Up to this point, I’d felt like I was struggling to see differences in shoulder slope between the horses I was familiar with.
I realize this thread is about dressage performance, but I think there are things that both top jumpers and top dressage horses would share, compared to average horses.
But it’s also true that a big, beautiful trot is the signature of a good harness horse. Some Standardbreds come with a natural trot that looks like it would be a huge winner in dressage, but then they don’t find canter easy, or collecting easy. They are more full steam straight ahead. So I assume one challenge is breeding dressage horses out of older harness stock would be keeping the trot, but adding on some ability to canter and collect. Warmbloods certainly don’t collect or turn or handle the way Iberian horses can, but then Iberian horses don’t have quite the size of trot of modern warmbloods.
The other thing that contributes to float is high energy. Even my Paint mare can passage, for about 30 seconds, when she’s first let loose with her friends in pasture!