Ok....this is all assumptions based on the VF-1 and VF-0 sculpts.
Improvements on the YF-19
1.) Legs will be connected to the torso with ball joints with more or less ample move space to allow dynamic posing in battroid.
2.) Knee joints will be improved and can swivel to allow better poses in both battroid and Gerwalk.
3.) this may be a long shot, but from the CAD i can see that the torso could very well be made to swivel. The air intakes may get in the way though.
4.) Head should be put on a ball joint. I never understood why the VF-1 and VF-0 's heads weren't put on ball joints as it improves the head's poseability but doesn't compromise in transformation.
And as for "features":
1.) The "wings swept back" could be done if there was a cavity within the YF-19's legs that allows some clearance for the wings to fold back. This may compromise the toy's look though. But another way is to make the wings double jointed so you would have to pull out the wings 1s (still connected by the joints to allow perfect transformation), fold it, then snap it back into place.
2.) I agree totally on the shoulders looking too flat. In fact it doesn't look like it has improved over the Bandai VF-19Kai ( i don't have the 1/72 yammie so can't make the comparison) and the shoulders are more or less static. What could greatly improve the look is to put the shoulder itself on a ball joint or a 2dimensional joint, so the shoulders are more like shoulder pads instead of a whole piece by itself. This way, the shoulders can be angled upwards in battroid mode but can also be angled downwards in fighter mode for a sleek fighter.
Another way is to make the shoulder joints able to rotate up by 10 to 20 degrees. This would add an extra ratchet joint to the top of the torso though and possibly would look a tad ugly if viewed from the top.
OR they could make the top of the torso itself slightly angled so when whe shoulders move into place, it would naturally be angled upwards.
And if they could make the shoulder swivel a little to the front as opposed to stopping right at the sides, to allow the hands to reach in front of its chest, that would be added cool to poseability.
Here's my lousy photoshop of how the angled CAD would look like.