I have used a microcontroller from ST, very similar in size and also with a Cortex-M0+ core.
The applications required only I2C to communicate with a bunch of other integrated circuits and a few general-purpose pins.
An example of an application was a kind of hardware video converter, which received video input from a camera and then sent it wirelessly or on cables, where the MCU configured and reconfigured everything on the board, after reset or when certain buttons were pressed, and the configuration for some things, like a HDMI transmitter, was complicated, requiring the reading and the writing of many internal registers via I2C, so a MCU was really needed.
There are many types of complex integrated circuits that need to be configured with values written in internal registers to be usable, so, even if just for the initial configuration after reset, you need some small MCU that can write the registers via I2C or SPI. For this, the smaller the MCU is, so it will not take space on the PCB just for booting the other ICs, the better.
Anything where a reasonable person would say "this doesn't even need a microcontroller, just do it analog".
In terms of projects that I would be inclined to try that are uniquely enabled by this, my mind goes towards wearables. It's small enough to be hidden in seams of fabric. If you wanted to have a bunch of temperature sensors all over your body, or have a complex arrangement of dimmable LEDs woven into clothing, each string going to its own controller hidden close by and communicating with a central controller, or maybe measure your skin conductivity all over your body or something, this is a great piece of tech to do that with style. You still need to run power and data wires everywhere (no RF on this chip), but flexible wires are a very solvable problem.
I made something to ping an AWS service to tell me the uptime of my internet connection. The idea was to sprinkle them around our area, connected to various home WiFi’s, and get a better triangulation of outages. Eg is whole pipe out, just one ISP etc.
I made and tested it but didn’t care enough to continue.
I have action camera with retarded user interface. To turn it on I have to: press main button for at least 0.3s but no longer than 2s, then I have to wait until it beeps, then I have to wait for 3s, then I have to press second button for at least 0.3s but no longer than 1s. Then I observe main led, if it blinks the camera is recording, if not I need to press second button again.
With tiny mcu like this one, I think it would be possible to add a bodge inside that would turn on recording automatically after single press of button. The MCU needs to be really tiny to fit inside camera.
It's got a UART, it's got a PWM, it's really fast, and it's got masses of memory. I'd build a really tiny synthesizer, port my Juno 106 plugin's voice engine to it.
The applications required only I2C to communicate with a bunch of other integrated circuits and a few general-purpose pins.
An example of an application was a kind of hardware video converter, which received video input from a camera and then sent it wirelessly or on cables, where the MCU configured and reconfigured everything on the board, after reset or when certain buttons were pressed, and the configuration for some things, like a HDMI transmitter, was complicated, requiring the reading and the writing of many internal registers via I2C, so a MCU was really needed.
There are many types of complex integrated circuits that need to be configured with values written in internal registers to be usable, so, even if just for the initial configuration after reset, you need some small MCU that can write the registers via I2C or SPI. For this, the smaller the MCU is, so it will not take space on the PCB just for booting the other ICs, the better.