Arrow: 7 Plot Threads The Show Totally Abandoned
3. Blue Beetle
One that wasn't the producers' fault whatsoever, Arrow had spent the first two seasons teasing the introduction of Ted Kord - throughout various references to Kord Industries - and the-powers-that-be were hoping to introduce him in Season 3. Unfortunately, that all fell through when DC informed them that they had other plans for the character.
On the plus side, this ended up blessing the Arrowverse with Brandon Routh's endearing iteration of Ray Palmer (a.k.a. The Atom) who ended up playing a significant role in the third season before jumping ship to co-headline five seasons of DC's Legends of Tomorrow.
That being said, it's still does leave us wondering if we'll ever see Blue Beetle show up in the Arrowverse.