Season 2 of Arrow lifted the "grounded" rule just a bit and gave us a super powered craze story line. Season 2 was also not afraid to introduce big name characters into the show. (Flash, Canary, Blood, Bronze Tiger etc.) Or even name drop and hint others. We're looking at you Harley Quinn. This is something season 1 hesitated to do and I believe that's why season 2 was so intriguing. Almost every other episode a well known guest character would appear. The Flash is going all out and have already cast major characters to appear. Arrow season 3 looks to be doing the same as we have Ray Palmer/The Atom, Ted Grant/ Wildcat and Ra's Al Ghul! Below is a list of some other characters that could appear this season.
(*Note, I've gone with characters that have not yet appeared in the series. So no Thea turning into speedy/cheshire and no Laurel turning into Black Canary)
The Question is a character that would fit right in on Arrow. He's more grounded than the Emerald Archer himself. Vic Sage is a journalist turned vigilante who after attaining a mask made of artificial skin, fights crime faceless. He's been depicted as a detective and also a conspiracy theorist. I could be wrong but, I don't think Arrow has actually ever had someone figure out his identity that actually has with no help. Question could be someone who comes to Starling determined to unravel the real truths behind all of Starling's terrorist attack, and also find out who the Arrow really is. Another way question could be introduced is by the Huntress. Ever since Helena was introduced I always thought she could be the way to bring in Vic. Because in the comics and most notably the animated universe they have a thing for each other.
In this episode the couple even fought Green Arrow and Black Canary.
A lot of people want Nightwing to appear in Arrow. But I couldn't really find a way for him to be introduced in the show. I opted for someone probably just as good and came up with a whole episode idea. With Roy Harper finally getting a suit and everything I now think Jason Todd would be a better choice. He's always been the most troubled Robin. (In his first appearance he tried stealing the Batmobile's Wheel) His introduction into Arrow could go a little like this. (*Note we will see Roy flashbacks not Oliver's this episode) Jason could be a childhood friend of Roy's and they both used to run around the glades beating up baddies. But see, Jason was a foster child, and after getting into to much trouble gets sent away to Gotham. Jason comes back to Starling to reconnect with Roy solely because he was the only true childhood friend he ever had, and wants to create a vigilante team with him. Jason even tries to lure him with wealth because he knows Roy struggled as a kid. But Roy sees how much Jason has changed, and that he now kills and steals, which is against Oliver's methods. BAM, insert a fight of Arsenal vs Red Hood and you have your Roy centric episode for the season. At the end of the episode though I think Jason should escape, but not before leaving the offer open for Roy to join his team. Foreshadowing the comic book team "Red Hood and The Outlaws".
I think Hawkman is another powered hero that could work on the show. I did make a whole article on him coming to [Arrow](series:720988) which included his origin and how he would be introduced etc. But to sum it up, have Katar (Hawkman) apart of an ancient tribe called Thanagar, that hides from the modern world because they have in their possession nth metal. Nth metal defies the laws of gravity, and they're afraid that mankind will destroy itself if they gained possession of it, which is why they hide. Katar Hol who is tired of hiding gains permission to go into mankind and explore. He crafts Nth wings and weapons and sets out. (Read more here)
Connor is one character who should not, I repeat not show up season 3. Yes he is Oliver's son, yes he does eventually become Oliver's sidekick, and yes Connor eventually becomes Green Arrow himself after Oliver dies. Now I've seen people comment about him showing up this year and being a competitor for Roy's spot as Oliver's sidekick. But remember, Oliver only had the child about 5 years ago; Connor is potentially 5-6 years old! Let's wait until season four, five or even six. If the Arrow writers want to surprise Oliver and introduce Connor this season that's cool. But you wouldn't get superhero Connor until season 7 or 8 providing the show goes on for that long. But if you guys want Connor Hawke the superhero this is not the right season. Right now it's Roy's time to shine.
Last but not least, The Vigilante! (He's the farthest to the left) Greg Saunders came home after his father (who was a sheriff) was killed. He later brought the men responsible to justice and battled crime as a western-themed hero. Greg is a superb hand to hand combatant, and an expert marksman. I mainly stuck him in here because Seven Soldiers of Victory is the very first superhero team Green Arrow joined, Roy was even in it! I think it would be cool to have a little nod to it. Since I can't say much about Greg, as he hasn't made an appearance in comics lately here's a video of all his moments in the hit show Justice League Unlimited. It showcases his personality and fighting style. It even features him fighting alongside Green Arrow and the Seven Soldiers of Victory. It should give you a good Idea of the character.