Niche mechanics are my jam. My favorites usually revolve around unique economies or structures that create their own depth of play.
I love genre benders and sandboxy stuff. I'd like to see something like Chaos Seed (SNES) built on or re-imagined, dwarf fortress is great for tinker-ability, and I always liked how Nethack has some deeply designed interactions even if they can seem superficial or esoteric. FFXII with the gambit and realtime-ish combat systems was always a standout too.
I'm honestly doubting it will get there someday, at least with the current use of LLMs. There just isn't true comprehension in them, no space for consideration in any novel dimension. If it takes incredible resources for companies to achieve sometimes-kinda-not-dogshit, I think we might need a new paradigm.