Currently DC has two separate universes running: The film universe, which will includes the recent Man of Steel and is set to add Wonder Woman and Suicide Squad to the list, and the CW universe, which takes place on the small screen, and kicked off with Arrow and it's spin-off The Flash. Now, we know what's happening in the films but we do not know what is in store for CW. Who's next? Crossovers? What will happen? Well, this article is here for speculation on what might be coming.
Arrow and Flash
I imagine that Arrow will generally remain the same. It's stayed solid for 3 seasons now and will keep doing well with new villains and new challenges to face. I would guess Ras will last to the end of the season then next time Oliver will get a new big bad. If I was to make a guess, I would think they'd move away from a physical opponent and more of an intelligent foe like Maxwell Lord or someone of similar esteem. A very powerful, much in the public eye businessman that tries to lead a campaign against the Arrow, whether that's openly or under the covers, hiring assassins and what not.
However, The Flash is quite different from Arrow because it's just gaining its footing but it's off to a very strong start. It's most likely gonna finish the current plot of Reverse Flash by the end of the season but with potential for a return, and with guaranteed time-travel elements (something only X-Men has live-action attempted so far), meaning if they pull it off, season 2 will open doors to much whackier plots and villains possibly, of the more super-powered kind which wouldn't be appropriate on Arrow. A priority of which, for me personally, is the Mirror Master. He is up there with Captain Cold and Reverse Flash as the most iconic and dangerous Flash villains.
Obviously crossovers are going to become a more and more regular thing between the two parallel shows, thanks to the success those episodes have had so far, and we expect to see at least one on each show every season.
Likely Possible Spin-Off's
Debuting in Firestorm, the Nuclear Man No. 1 in 1978 and has been a DC comic staple ever since being a popular member of the Justice League. Ronnie finally made his way on to the screen in The Flash and is expected to become a much bigger part of the second half of the season, thanks to his intervention in the big fight between Flash and his reverse counterpart. Firestorm is one of the biggest contenders for a spin-off since he is a big name at DC, but not quite big enough for a film (much like Green Arrow) and has already effectively used the entirety of the Flash season as a back-door pilot. It plays along much like The Flash but more independent (not back-up teams like Arrow and Flash), and a bit grittier.
The Ray Palmer Atom debuted 1961 and is in pretty much the same position as Firestorm, a popular Justice League member using an existing show as a backdoor pilot, and a prime candidate for the next spin-off. Although, as of yet, we haven't seen him suited up fully into his exoskeleton-esque outfit (which Arrow is relentlessly teasing). Now, I'm not an expert on The Atom so I don't know really what he's all about, but I'm sure it'd make for a very interesting show.
Birds of Prey
The Birds of Prey are an all-female comic-book team largely based in Gotham, which was fundamentally based around Oracle and Black Canary. It has included many heroines from DC comics such as Huntress, Lady Blackhawk, and Dove. Arrow had an episode entitled "Birds of Prey" which, although it didn't include the team, did include Black Canary and the return of Huntress. If you add this to the rumor that Felicity will develop into the CW rendition of the Oracle, it makes up the team nicely and even though Sara is dead Laurel will be taking up the mantel. Don't forget Katana is also coming to Arrow, so hopes are high. The series would most likely document the team taking on some jobs in other places away from Starling City which Oliver can't investigate but would very much be an independent show of the same tone as Arrow.
My personal favorite comic book team is the Suicide Squad, a top secret black-ops unit assembled by the government because of their special skills and complete expandability to carry out some of the most dangerous missions known to man. The Suicide Squad featured in their titular episode in Arrow Season 2, an is one of the highest rumored for a spin-off, thanks to it's positive fan reception. This incarnation includes/has included: Deadshot, Bronze Tiger, Shrapnel, Harbinger, Cupid, Captain Boomerang, and briefly John Diggle. This would make for a very interesting series but is now likely not going to happen thanks to the upcoming David Ayer film.
Less Likely Spin-Off's
He is a slightly more obscure DC character which the public don't know much about, but he is coming to Arrow soon and has in fact been cast as J.R. Ramirez. It's unlikely to go anywhere but still just possible.
Katana is coming to Arrow this season to be played by The Wolverine star Rila Fukushima after Devon Aoki left. This is a really great character in the comics and if all goes well in Arrow she could pull off getting her own show. It would be similar to Arrow, but with a touch more mysticism.
Now, S.T.A.R Labs is an integral part of the Flash show, but in the comics did it not play a key part in the creation of a major hero? Yes. Cyborg. Cyborg in the New 52 was a founding member of the Justice League after he became half machine when his father attempts to save his life. The origin story in Flash could be changed to something more along the lines of Victor's life being in danger, which Flash is sort of responsible, for he so takes him back to S.T.A.R Labs where, in order to save his life, they combine him with technology.
Ferris Air has appeared in multiple episodes of Arrow and the pilot of The Flash. Ferris Air in the comics was owned by Carol Ferris (Star Sapphire) and the company that employed Hal Jordan (Green Lantern) as a pilot. Again this is just a minor reference but could we be seeing more of Green Lantern?
Highly unlikely thanks to an executive producer basically saying Batman and Superman are off limits to the shows, but it did say Wayne Tech in the article from the future.... If it were possible I doubt it would happen because it would be much too similar to Arrow.
This is a big fan-favorite for a spin-off. However, it's unlikely thanks to Batman being banned; but Steve McQueen along with Steven Amell (Arrow) have been teasing for a while that Steve has been cast as Nightwing. Nothing is confirmed, but Arrow does constantly mention Bludhaven which is the hunting ground of a certain ex-Robin...
Practically Impossible Spin-Off's (But We Can Hope)
It was said that before Ray Palmer the guys at Arrow wanted to use Ted Kord, but apart from that there hasn't been much reference. Also his good buddy.....
Because why not? He's an awesome character who can time travel and has a robot friend. Who wouldn't want to watch some Booster Gold hijinks?
Again, why not? A rich backstory and high fan support, it's worth a try since they're not going to be getting the movie treatment any time soon.
This is my personal favorite for a spin-off. She's a wonderfully quirky character who is a fan-favorite without the hope of a film. She would't be dependent on either of her sister series' which handle crime-lords and super-criminals. She could handle a magical side of things, which would be really great to see.
And Then Finally, What Could Happen In The Future Of The CW Universe....
The God-Damn Justice League!
With CW definitely creating their own universe and trying to spin it off any which way they can, is it any surprise that they are most likely creating a Justice League for the small screen, much like Smallville did? But the great thing is that it might actually work. If you work off the original Superman, Batman, Wonder Woman, Flash, Aquaman, Green Lantern and Martian Manhunter/Cyborg, then a lot of the slots can be filled and Flash would take his rightful space; Arrow would step in for Batman, Firestrorm as a Superman, Atom as an Aquaman, then the already possible Green Lantern and Cyborg take their spaces, then a female like Katana/Huntress/Black Canary to replace Wonder Woman. BOOM. CW Justice League everybody. Obviously this is a few years away and likely would not be its own series, which could be called together on an Arrow finale, but any way you put it, it's awesome.