Yeah, I agree that it's most likely going to be a FUNimation show replacing Deadman Wonderland. I don't really see what Aniplex USA show Adult Swim can pick up. I don't think Durarara!! did bad, but I don't think it did great either so I'm not so sure if Adult Swim is eager to acquire another anime series from them. IIRC, Kekkaishi did better than Durarara!!. I think there's more of a chance of a Viz show getting picked up than Aniplex, like Inuyasha: The Final Act.
I think when Ghost in the Shell ends, FLCL can take over since it's only 6 episodes. If not, I guess Ghost in the Shell can start from episode 1 and leave when Deadman Wonderland is done.
Aniplex has two series that have potential to do well on Toonami: Fate/ZERO and Blue Exorcist. However, Aniplex hasn't shown any interest in dubbing either series and in fact seems to be trying to prevent Western consumers from accessing the former series at all by making the DVD costs prohibitively high. This doesn't mean that Aniplex
isn't dubbing either series though, Aniplex kinda kept the DRRR!! dub a secret right up until release. Madoka didn't get the same treatment however, Madoka's dub was announced well before release. So it could fall either way.
FLCL could probably hold down the fort for a few weeks, I would not be shocked to see that show make a one-and-done run as a placeholder to buy time for greater, newer things. As for Viz, I think Inuyasha's chances have become remote at best . . . I think AS is moving on from Inuyasha and they don't seem to think that The Final Act would do well enough to justify the asking price. Tiger & Bunny which has massive mainstream potential would likely be the smarter choice. Viz thinks so too considering they're spending big bucks on the dub (just check out the announced cast so far).
The question if it's a FUNimation show is whether it's something new whose DVD release is a while away (A Certain Magical Index, Steins; Gate, Shangri-La) or something a bit older that was under the radar like Casshern SINS was (Shakugan no Shana, Gai-Rei ZERO, GunXSword, about a zillion others). Keep in mind that if FUNi sold their shows in packages, Fullmetal Alchemist: Brotherhood was renewed a couple of months ago and there's a possibility FUNi attached a show to the deal to continue running Brotherhood. Whether that show is already on the air or hasn't yet doesn't really matter . . . but at the same time I wouldn't hold my breath, it's just as likely that FUNi didn't attach anything to the deal.
I would not discount a Toonami classic from showing up in one of the later slots either. In all likelihood it'll be something owned by FUNi, like Yuyu Hakusho, considering how much of a withered remnant Bandai Entertainment is. But Trigun got featured on April Fools' Day and the powers that be might find it appropriate to give the show a run.
It could be possible that Fate/ZERO or Blue Exorcist could be picked up, but I still think that a Viz or FUNimation series is way more likely. 
But seriously though, cudos to Combuskenisawesome for the Toonami templates! He truly is awesome!
