NordicRF (the company behind nRF24L01) has released two series of SoC devices that are compatible with nRF24: NRF51 and NRF52. Thanks to the effort by Sandeep Mistry and other contributors, they can be programmed using the Arduino IDE and libraries. Instructions are available here: https://github.com/sandeepmistry/arduino-nRF5
I have tested NRF51822, which is the cheapest variant (costing slightly more than Arduino Pro Mini + NRF24L01+ but in a single small and power-efficient package) and it works great.
The BBC Micro:bit is based on NRF51. More information is available at https://forum.mysensors.org/topic/9722/bbc-micro-bit-and-mysensors-a-great-match
Compared to the Atmega328-based Arduino Pro Mini, NRF51822 has 8x larger flash and 8x larger RAM. It also has Bluetooth 5 and BLE support and a 32-bit ARM Cortex M0 CPU.
There is also this alternative: https://www.ebid.net/eu/for-sale/arduino-ide-compatible-controller-with-the-nordic-nrf24l01-radio-transceiver-156695123.htm
- Dimensions 59mm x 18mm
- The Nordic nRF24L01+ 2.4 GHz Radio transceiver on board
- Supply voltage up to 6.5 Volts
- The Digital and Analog pins are 5 volts
- ATMega328P frequency is 8 Mhz
- 4 battery connector pins
- FTDI header for programming
Full disclosure: I am member of the MySensors core team. A MySensors user (not affiliated with the core team) created and is selling this board.