  By deploying antenna arrays at the transmitter/receiver to provide additional
spatial-domain degrees of freedom (DoFs), multi-antenna technology greatly
improves the reliability and efficiency of wireless communication. Meanwhile,
the application of multi-antenna technology in the radar field has achieved
spatial angle resolution and improved sensing DoF, thus significantly enhancing
wireless sensing performance. However, wireless communication and radar sensing
have undergone independent development over the past few decades. As a result,
although multi-antenna technology has dramatically advanced in these two fields
separately, it has not been deeply integrated by exploiting their synergy. A
new opportunity to fill up this gap arises as the integration of sensing and
communication has been identified as one of the typical usage scenarios of the
6G communication network. Motivated by the above, this article aims to explore
the multi-antenna technology for 6G ISAC, with the focus on its future
development trends such as continuous expansion of antenna array scale, more
diverse array architectures, and more flexible antenna designs. First, we
introduce several new and promising antenna architectures, including the
centralized antenna architectures based on traditional compact arrays or
emerging sparse arrays, the distributed antenna architectures exemplified by
the cell-free massive MIMO, and the movable/fluid antennas with flexible
positions and/or orientations in a given 3D space. Next, for each antenna
architecture mentioned above, we present the corresponding far-field/near-field
channel models and analyze the communication and sensing performance. Finally,
we summarize the characteristics of different antenna architectures and look
forward to new ideas for solving the difficulties in acquiring CSI caused by
the continuous expansion of antenna array scale and flexible antenna designs.
