When I first saw Rusev, I chuckled and didn't take him seriously - and I still don't. Without Lana, I think he is mostly dead in the water. That shaking fist thing he does at the end of his matches (like he is shooting a gatlin gun maybe?) makes him look like a giant baby to me - I guess that is neither here or there.
I have to admit the pair have grown on me though and much to my surprise, the crowd has jumped onto the old "USA USA USA" chanting of old. Does the gimmick have legs beyond his feud with Swagger? I think if they feed him patriotic legends it will get him a few more months. Beyond that...
I think the only other place to go with Rusev is to change him from Anti-American to Global Dominator. Something along the lines of "he has defeated America's best hope, now he is going to take over the world." From there it makes sense for him to battle the likes of Shaemus and other foreigners, as he "invades" the rest of the roster. Meanwhile, in the background, Swagger or some other "American Hero" trains and prepares for round two, culminating in an eventual victory.
But again, I don't see it lasting beyond that. When you have such specific gimmicks, I think it is hard to make any sort of real superstar out of them - with the exception of badass supernatural characters. Fandango, Rose, Rusev - so long as they are gimmicks and not real people, they'll never last. It's only when a wrestler really incorporates aspects of a character into themselves (and not the other way around) that they make it. Cena, Stone Cold, The Rock, Daniel Bryan, Punk - they have a little gimmick, but mostly they are just themselves - and that, to me, is the difference between a "superstar" like Rusev and a Legend like Jericho.
That and just the natural IT factor of course...