Can MGF1 within OAEP and PSS be replaced by a XOF?
It seems that the authors of Keccak sponge (the algorithm chosen to be SHA-3) do think their SHAKE functions can directly be used for simplifying OAEP and PSS:


  The introduction of extendable-output functions (or XOFs, pronounced zoff) is a particularly nice feature of the standard. A XOF like SHAKE128 or SHAKE256 can be seen as a generalization of hash functions where the output length is not fixed but is potentially infinite. Concretely, XOFs can be used instead of complex constructions involving hash functions and counters such as MGF1. With RSA, this is of immediate benefit to full domain hashing (edit: also known as FDH), to RSA OAEP (Optimal Asymmetric Encryption Padding) and to RSA PSS (Probabilistic Signature Scheme). Other use cases are key derivation functions and stream ciphers.