Zangetsu always felt like the ultimate companion to Ichigo's journey, but the romance fics exploring their bond really dig into the loneliness and yearning under the surface. The most popular plot I've seen is the 'inner world confession' scenario, where Ichigo, after a brutal battle or a moment of deep doubt, finds himself lingering in that endless skyscraper landscape. Zangetsu, usually the Old Man version, stops being just a teacher and starts voicing his own hidden attachments. It shifts from mentorship to something painfully intimate, and Ichigo has to grapple with loving a part of his own soul. Another common one is the 'separation and reunion' arc, where some magic or Quincy artifact temporarily splits them. Ichigo is left feeling hollow, literally, and Zangetsu has to navigate having a physical form in the human world. Watching them orbit each other, confused by human emotions but fiercely protective, never gets old. It's all about that tension between being one being and two distinct souls craving connection.
I'm less keen on the outright 'Zangetsu is secretly a hot guy in a cloak' AUs that ditch the spiritual aspects. The appeal for me is the metaphysical romance, the self-love coded as epic partnership. The good fics make you feel the weight of all their shared battles and silent understanding.