Interestingly, Arrow's milestone 100th episode will fall right in the middle of The CW's four night crossover which will also include Supergirl, The Flash, and Legends of Tomorrow. Now, a photo Tweeted out by executive producer Marc Guggenheim reveals that Jamey Sheridan is going to return as Oliver Queen's long dead father, Robert. However, it won't be part of a flashback of the Flashpoint.
Throw in the fact that Katie Cassidy has confirmed on Facebook that she will also be making a return, and chances are that time travel is going to be a big part of this crossover. Seeing as she signed a deal to become a regular across all of The CW's DC Comics shows, her return was to be expected, but it should be very interesting to see how these two end up factoring into Arrow's 100th episode...