A new 3GPP Technical report, TR 38.858 (draft not available yet) will look at Study on evolution of NR duplex operation (FS_NR_duplex_evo) in Rel-18. RP-213591 provides a justification on why this new duplex evolution needs to be studied.
Read more: The 3G4G Blog: Extending 5G TDD Coverage With XDD (Cross Division Duplex)