Most important grey skills
There are white skills and grey skills, but a skill being grey doesn't mean it's completely useless for a player of specific position, just as not all white skills are equally important. The developers at some point had to decide where to draw a line on which skill being white and which gray, even if there weren't a big difference between the least important white and most important grey.
This might be grasping for straws, but more relevant now after than before the training update. You can't use solely the drills with white skills anymore. So, if you have to put some emphasis on grey skills for players, what would they be? I'd say:
Heading for full backs, you can see in the match animation that it's not uncommon for DL & DR to be defending crosses against opposing attackers. Wide players tend to improve in heading inadvertently now since most crossing drills have heading and grey skills gain as much as whites in training.
Speed for DC, they're often defending against attackers attempting a breakthrough. They don't have to even nearly have as high speed as attackers since the ball-less player has advantage, but I wouldn't want my DC have some 50 or less speed. :D
Positioning for AM's. Positioning is kind of hard to judge since it's more of an indirect skill, but you can see AML/AMC/AMR getting played into scoring chances like being in the receiving end of through passes, so positioning skill likely affects AM's too.
