Dont think he impressed enough at the world cup to get a contract in anything but the lower leagues sadly. That stint at Celtic really stalled his career, he was going great at Hibs.
I get the impression Killen would rather sign for a big club without getting much gametime. What's to say a team like Hibs wouldn't want him back.. all he needs is one season where he finds the back of the net reguarly and he'll be set again.