"Cover me!!!"
-- many, many action-movie characters
I think a ranged defender should be perfectly doable, and different from a controller. A defender draws fire away from his teammates, particularly the squishy wizards and low-Con strikers. So a defender needs powers that make him a more attractive target; this seems different from the mez powers and battlefield control of a wizard (who makes it hard to attack anyone, including himself).
How about this class feature:
Suppression
When you target a foe with a ranged attack on your turn, they become marked until the end of your next turn. A marked target who charges you, attacks you, or moves towards you, does not provoke any opportunity attacks from anyone for doing so. If a marked target within 12 squares of you and within short range of your ranged basic attack performs any other action that provokes opportunity attacks, you may make a ranged opportunity attack against them. You don't provoke opportunity attacks from a marked target.