As the book ends, Ivan and his animal friends no longer live lives in captivity in the mall. Ivan and Ruby now reside at the zoo, while Bob has found a home with George and Julia. It's about as happy an ending as anyone could hope for.
However, Ivan stall has some hurdles to jump, the biggest being his status as a silverback gorilla. Genetically speaking, silverbacks are meant to be leaders, and Ivan hasn't had much of an opportunity to be a dominant figure in his life. He's a gentle guy, a sensitive artist. So while he loves the idea of beating his chest and having everyone run in the other direction, the reality is that he's a thinker, a tree-branch philosopher without a lot of alpha male impulse.
So when he whispers "Mighty silverback," it leaves a few questions for readers to ponder. Is Ivan feeling unworthy of the status? Is he ready to step into this new role? This ending leaves the door open enough for readers to wonder if Ivan still wants the part, or is shyly announcing that he's already got it.