Deathstroke

Everyone loves a great villain, which explains Deathstroke's popularity. He's a mercenary, and he's able to go toe to toe with Batman. His character has everything that he needs to be awesome, and he's already appeared on Arrow. If DC is looking to expand their TV multiverse, Deathstroke is an easy choice. This is probably the perfect time to capitalize on him, mainly because of Deadpool, as the original comic version of Wade Wilson was kind of a ripoff of Deathstroke (named Slade Wilson). While Wade started getting funnier and breaking the fourth wall over the years, Deathstroke has been waiting in the shadows. We'd love to see Deathstroke in his own Arrow spin-off series.