For example, by viewing Hilbert spaces as enriched categories in some fashion? (I suppose the same idea of considering the inner product of a Hilbert space as a generalized Hom-set has also been pondered in connection with the similarity of adjoint transformations and adjoint functors, though I'm not aware of any more formal correspondence there either)
Edit for clarification: The full similarity I see is this:
For any Hilbert space $H$, we have its inner product, a continuous linear map from $H^{op} \otimes H$ to $\mathbb{C}$ [where $H^{op}$ is $H$ with its inner product's argument order flipped]; currying this gives a continuous linear map from $H^{op}$ into the Hilbert space of continuous linear maps from $H$ to $\mathbb{C}$. The Riesz representation theorem says this is an isomorphism.
Similarly, for any category $H$, we have its Hom functor, a continuous functor from $H^{op} \times H$ to $Set$ [where $H^{op}$ is $H$ with its Hom functor's argument order flipped]; currying this gives a continuous functor from $H^{op}$ into the category of continuous functors from $H$ to $Set$. The Yoneda embedding lemma says this is an embedding; furthermore, under suitable conditions (e.g., if $H^{op}$ is equivalent to a presheaf category), this is an equivalence.
