What should we add?

There are several classes of circuit transformations that take a single circuit in and create output over many circuits. Examples include Pauli Twirling and Lightcone truncation over a linear combination of operators. It is tempting to write these circuit transformations as transpiler passes, but they do not satisfy the current model of one circuit in, one-out for passes. Currently, the best way to implement these methods is to break transpilation into 2 pieces and perform the one-in many-out operations in-between. It would be cool if the transpiler could accommodate these types of transformations