I did think of hand size and ultimately concluded that so many traits are already tied to it (carry, catching, snapping, awareness for bad handoffs, hand fighting to an extent), that ultimately I didn't include it in the end.
That was how I imagined first step, short area explosiveness and reaction time off the snap! Goes beyond just straight acceleration.
By the time you factor in Stamina, Season Stamina, Mass, Finish plus other heart & physical ratings I felt you had a decent picture of the effort level of a player to play up to their ratings.
Love the idea of it all being tied to coaching (and scrapping XP), teams would not only scout/draft based on their preferences, but each team would coach players differently. The progression, regression and potential ratings determining how easy and how much a player can be coached.