  Recent measurements at the T2K experiment indicate that CP violation in
neutrino mixing may be observed in the future by long-baseline neutrino
oscillation experiments. We explore the physics program of an extension to the
currently approved T2K running of $7.8\times 10^{21}$ protons-on-target to
$20\times 10^{21}$ protons-on-target,aiming at initial observation of CP
violation with 3$\,\sigma$ or higher significance for the case of maximum CP
violation. With accelerator and beam line upgrades, as well as analysis
improvements, this program would occur before the next generation of
long-baseline neutrino oscillation experiments that are expected to start
operation in 2026.
