  The Murchison Widefield Array (MWA) is an open access telescope dedicated to
studying the low frequency (80$-$300 MHz) southern sky. Since beginning
operations in mid 2013, the MWA has opened a new observational window in the
southern hemisphere enabling many science areas. The driving science objectives
of the original design were to observe 21\,cm radiation from the Epoch of
Reionisation (EoR), explore the radio time domain, perform Galactic and
extragalactic surveys, and monitor solar, heliospheric, and ionospheric
phenomena. All together 60$+$ programs recorded 20,000 hours producing 146
papers to date. In 2016 the telescope underwent a major upgrade resulting in
alternating compact and extended configurations. Other upgrades, including
digital back-ends and a rapid-response triggering system, have been developed
since the original array was commissioned. In this paper we review the major
results from the prior operation of the MWA, and then discuss the new science
paths enabled by the improved capabilities. We group these science
opportunities by the four original science themes, but also include ideas for
directions outside these categories.
