Historically, AirPort Extreme and Time Capsule are refreshed every 2 years and AirPort Express every 4 years. Based on that, AirPort Extreme and Time Capsule are now due for update. At the very minimum, they need 4x4:4 (transmit antennas x receive antennas : streams) MU-MIMU (current products are 3x3:3) and USB 3.0 ports (USB-C perhaps). I am also crossing my fingers for QoS (which would improve Wi-Fi Calling), iTunes media hosting (at least for Time Capsule), and VPN server, but I strongly doubt it. If you ask me, Apple should just eliminate AirPort Extreme for good and sell only Time Capsule and AirPort Express (updated for 802.11ac).