I don't see him dropping it anytime soon. As of right now, I'm just wanting WWE to move him on past Miz & Big Show, though I do like the fact that they've been putting emphasis on IC title feuds and storylines since WrestleMania. The problem is that there've been some set backs such as Daniel Bryan having to take time off due to a concussion and Ryback being out due to a Staph Infection. Another problem is that, frankly, I don't think a ton of people are overly excited about Miz and Big Show being the challengers. However, the upside of that is that at least they've kept the emphasis on the title with Show & Miz while Ryback's recovering from the infection, which is quite a bit more than they were doing 6 months ago. Also, Ryback dropping the title anytime soon sort of undermines the whole notion of giving the mid-card titles a fresh start and the IC title itself has a helluva lot of catching up to do when you compare it to where Cena's helped bring the US title.
I could see Rusev as a viable candidate, though I think it might depend upon how he comes out of his feud with Dolph Ziggler. Ziggler, as we know, was written off TV a little while back due to suffering a "crushed larynx" after Rusev injured him with a crutch, though he's actually been out shooting a movie and could be done with it in time to face Rusev at SummerSlam. If Rusev comes out on top in his program with Ziggler, then I think he'll definitely be on the short list.
Another possibility is Kevin Owens, though there's also still a possibility of him rekindling his program with Cena somewhere down the line over the United States Championship. Owens has made quite a splash this summer during his feud with Cena, though there's also some sort of possibility that he & Cesaro could be in the WWE WHC picture if Cena's gonna be out for a little while.
Speaking of Cesaro, he's another potential candidate though it's more unlikely as he's a babyface at the moment. He's had some really good showings in some matches in the past against Ryback and it's fun seeing Cesaro match strength with Ryback; we don't usually see Ryback being tossed about or manhandled unless it's been by someone like Big Show or Mark Henry.
As to who should take the title right now, as I said, I think Ryback needs to hold onto the title probably for the rest of the year, maybe drop it at the Royal Rumble to someone but I'm mostly just interested in seeing Ryback continue his run to see how things go. Ryback's grown in popularity and he's pretty good in the ring, especially for a big bruiser, though I'd personally love to see him adopt a more interesting persona than this gym rat/meathead thing he's got goin' on.