The Adafruit Feather 32U4 seems like the modern successor to the Arduino. It has a slightly smaller form factor (22.9 mm × 50.8 mm), comes with a LiPo battery-charging circuit, weighs 4.8 g, and it’s based on the ATmega32U4 at 8 MHz at 3.3V. This is a step down in computational power, but also in power consumed. But it has 20 GPIOs, 7 PWM pins, 10 analog inputs, a prototyping area, and its pin spacing is breadboard- and perfboard-compatible; and it supports USB directly, so you can directly do things like keyboards.
And of course it’s software-compatible with Arduino, although it’s running at half the cock speed.
Instead of “shields”, the Feather has “wings”, which plug into optional female headers you can solder on. There are a few dozen wings available, including things like Wi-Fi, OLED displays, etc. I²C is the primary means of communication between the Feather and its Wings in order to get a limited amount of stackability.
There are also Feather base boards with other CPUs, apparently including a Cortex-M0 with Wi-Fi (an ATSAMD21 and ATWINC1500, US$35), a Cortex-M4, an ESP32 (“Huzzah32”, US$20), an ESP8266 (“Huzzah”, US$17), and an STM32F205 with Wi-Fi (“WICED”, US$35).