This is the main issue for adding new features to the existing SPI API.

• Adding 3-wire support (half/full duplex modes) See SPI API: Adding 3-wire mode support #38386
• Adding frame format selection (TI vs Motorola) See SPI API: frame format configuration option (TI vs Motorola) #39990
• Adding proper support for dual/quad/octal modes See SPI: Add API support for DDR, Dual, Quad and Octal modes  #39991
• Change cs_lock/lock_on and spi_release() See SPI API: spi_release() and lock_on/cs_hold operation bits #40170
• Constify struct spi_config (API and all instances). API is still set to use constant struct spi_config, but let's verify that all instances are constant as well
• Updating DTS bindings to expose these new features (this probably needs to be done in the relevant PRs above)
• Improving asynchronous methods See SPI: async methods update #43102