Arrow's going to have a lot of... time on his hands in the second half of the season with the Clock King showing up to give the Arrow a run for his money. William Tockman will be portrayed by none other than Robert Knepper whose also busy being on Hunger Games: Mockingjay.
The description for The Clock King:
"a brilliant criminal who plans his crimes with the utmost precision, down to the second"
Sounds like he'll be more of the clock based villain and possibly come back as the I can see 2-3 seconds in the future villain and beat up some very skilled people.
Clock King will make it in the 14th episode which will guess what..... be just in time for Black Canary to come back!!