The best replacement for metaphysics is epistemology.

The best alternatives to metaphysics are rated as following:
★★★★★  epistemology
★★★★  ontology
★★  physics
★★  quantum physics
★  dialectics
  quantum mechanics
  axiology
  existentialism
  aristotle metaphysics
  phenomenology

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Speaking about metaphysics, one probably shouldn't forget about philosophy.

Other search terms related to metaphysics and their ratings are:
★★★★★ philosophy
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metaphysics
epistemology
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