Handle measurement in circuits
It needs to possible to handle projective measurement in a circuit--generally this would be implemented as a special case of the Z gate, but the exception that implements some extra bookkeeping to mark a measured qubit as a "classical" bit for the remainder of the circuit.
Evaluating circuits containing measurements should be simple enough, thanks to the principle of deferred measurement. But there could be some trickiness involved if, for example, a classical bit as used as input to a gate that expects a qubit and might try to put it into a superposition or otherwise modify it. After that point it should no longer be treated as classical. This case could be difficult to determine a priori such as when generating a circuit diagram. But it's also an unusual use case I think and not something to worry about too much.