  KAGRA is a new gravitational wave detector which aims to begin joint
observation with Advanced LIGO and Advanced Virgo from late 2019. Here, we
present KAGRA's possible upgrade plans to improve the sensitivity in the decade
ahead. Unlike other state-of-the-art detectors, KAGRA requires different
investigations for the upgrade since it is the only detector which employs
cryogenic cooling of the test mass mirrors. In this paper, investigations on
the upgrade plans which can be realized by changing the input laser power,
increasing the mirror mass, and injecting frequency dependent squeezed vacuum
are presented. We show how each upgrade affects to the detector frequency bands
and also discuss impacts on gravitational-wave science. We then propose an
effective progression of upgrades based on technical feasibility and scientific
scenarios.
