Either way, the sentence is correct. Some would also prefer the following form of the sentence: I can only hope to someday become as great as you. This more definitely connects the "someday" to your "hope." Just so you understand, the "are" is implied in the term "as great as you" which is why it is not necessary, it is inferred by the reader that the person is currently great.

I can only hope to become as great as you (are) someday. You could omit the "are", but it sounds nicer with it.