  Recent research on proactive conversational agents (PCAs) mainly focuses on
improving the system's capabilities in anticipating and planning action
sequences to accomplish tasks and achieve goals before users articulate their
requests. This perspectives paper highlights the importance of moving towards
building human-centered PCAs that emphasize human needs and expectations, and
that considers ethical and social implications of these agents, rather than
solely focusing on technological capabilities. The distinction between a
proactive and a reactive system lies in the proactive system's
initiative-taking nature. Without thoughtful design, proactive systems risk
being perceived as intrusive by human users. We address the issue by
establishing a new taxonomy concerning three key dimensions of human-centered
PCAs, namely Intelligence, Adaptivity, and Civility. We discuss potential
research opportunities and challenges based on this new taxonomy upon the five
stages of PCA system construction. This perspectives paper lays a foundation
for the emerging area of conversational information retrieval research and
paves the way towards advancing human-centered proactive conversational
systems.
