  The present document outlines a roadmap for future developments that will
significantly expand ALMA's capabilities and enable it to produce even more
exciting science in the coming decades. The proposed developments are motivated
by the groundbreaking results achieved by ALMA during its first five years of
operation. The roadmap described here is based on input on new scientific
directions and technical feasibility of future developments from the ALMA
Science Advisory Committee (ASAC), the community, and technical documents.
  The Working Group recommends that the top development priority, based on
scientific merit and technical feasibility, is to broaden the receiver IF
bandwidth by at least a factor two, and to upgrade the associated electronics
and correlator. These developments will advance a wide range of scientific
studies by significantly reducing the time required for blind redshift surveys,
chemical spectral scans, and deep continuum surveys. In order of scientific
priority, receiver upgrades are recommended for intermediate (200-425 GHz), low
(< 200 GHz), and high (> 425 GHz) frequencies.
  The Working Group recommends that the receiver and throughput developments
proceed as soon as fiscally and technically feasible. As a first step, a
technical and scientific group should be formed to formalize the top-level
requirements. A team of systems engineers should then be charged with flowing
these requirements down to the subsystems to form a consistent new set of
minimum requirements, which future development projects would have to meet.
Given that upgrading the throughput will impact many ALMA subsystems, the
Working Group recommends that a team within ALMA be charged with coordinating
and monitoring these developments. (Abbreviated)
