This is actually one of the things we're looking at with the open-source lowRISC project. Moving aspects of the I/O system to RISC-V cores to enable you to implement SPI, I2C, I2S etc and other custom protocols in software. http://www.lowrisc.org/docs/memo-2014-001-tagged-memory-and-...

For integration of more analog components, I think the Silego GreenPak is a very interesting product http://www.silego.com/products/greenpak3.html.