I think Robin Hood will show up in Storybrooke and come forward to help Belle run the town because she once helped him. I’m hoping he will become acting sheriff because Storybrooke really needs another hunky sheriff to take care of business. That makes sense to me because Belle is the only character Robin has interacted with besides Rumple. Also, Emily de Ravin is a good actress but not strong enough to carry the home front storyline by herself and the actors portraying the minor recurring characters are not much help there so a strong actor in a prominent role would help the storyline.
The casting call for Robin Hood makes sense if they are planning to continue his FTL storyline and have Robin return to the Dark Castle where Belle keeps giving him food and supplies behind Rumple’s back, or Belle uses her flirting skills to coerce Rumple into helping Robin against his will, or something like that.
Or they just needed to hedge their bets with the casting call in case they couldn’t get Tom Ellis in time.