Multimodal logic with quantification over modal operators
In topos theory there are modal operators known as the Lawvere-Tierney operators. Such an operator is a certain endomorphism $j : \Omega \to \Omega$ on the subobject classifier. The object $J$ of all such operators is definable in the internal language of the topos, and so we can quantify over all $j$'s. My topos-theoretic knowledge is too poor to know whether anyone has done anything interesting with the object of all $j$-operators.

A related idea is the frame (or locale) of nuclei. If we think of a frame as a kind of logic (geometric logic, to be precise) then a nucleus is a modal operator on it. Once again, the nuclei of a frame themselves form a frame.