Arrow Season 4: 8 Ways It’ll Be A Totally Different Show This Time
5. Time-Travel
The world of the show is expanding, largely due to the introduction of The Flash. That series started to introduce time-travel towards the end of its first season, and is going to continue exploring that in its sophomore year.
There's also the spin-off show Legends of Tomorrow, which is going to involve the time-traveller Rip Hunter, and will presumably tie into how Sara comes from the dead. All of these shows are linked and will crossover, so there's going to be a natural impact on Arrow.
While it hasn't introduced anything like time-travel yet, if it's happening in both of the companion shows, then chances are that Oliver at the very least is going to end up involved in it as well, and that it will have a direct impact on the show, really shaking things up from the world we thought we knew.