Must be currently playing ie. no Pele
Not a huge superstar that Arsenal can't afford ie. no Messi
You can stretch the reality a bit if you would like, maybe a player that costs $30mil or a guy that is on a rival team.
You could focus on talent, position of need (is there really anything else to focus on? A great blog? we already have meerkat.)
What player do you get that makes the difference for this squad?