[Crazy Jetty]

Being CW, it's probably going to be young, hip, and angsty.

If I had my way, I'd use Mia Dearden as an entry-point. Get a teen, or young actress to play her, have Ollie be already established as the urban legend known as Green Arrow, and like the comics, become a father figure to her.
Introduce the viewers to Green Arrow's origin, Roy Harper, and all their back stories like that through her.
He does need a perminant antagonist... I think the Queen from GA: Brightest Day may be good for that. She's rested Queen Industries from Ollie.
And have a rotating cast or mercinaries she hires, mob bosses, and independent villains.
I think Merline and Deadshot would make terrific adversaries for the first season. (She hires Merlin to assasinate Arrow, when he fails, she replaces him halfway through the season with Deadshot.
And while not being a traditional GA villain, I think Cheshire would make a great reoccuring character, especially given her history with Roy.