Ive thought about this too, how they could implement it.
In the end, I believe athleticism to be a combination of all existing non-shooting related stats.
eg: Handling + Stamina + Driving + ID + OD + SB + RB etc.
A modern day athlete i believe, is someone like Dwayne Wade. His ability to block shots is uncanny, and his ability to steal the ball, defend larger players, dunk of the rebound etc. Ok so im thinking highlight reel here but still.
If you took a C in this game, who has Level 18 IS, Level 18 RB and 18 ID.... But has atrocious passing./handling etc, it conjurs up images of a big hulking lug who lumbers around the court.
Looking at another C, who has Strong jump shot, and even just Average Handling/Driving etc, you start thinking of a more mobile athletic moving C.
Same with guards. ... a guard with decent rebounding, ID, SB, etc, it conjurs up images of a guard who can really jump and post up etc etc. Athlete.
So whilst i think its a good idea, if you use your imagination a little bit, you can find this athleticism already within the game. And strangely enough, i think you will find that those players who have excellent secondary skills are better players.