Another purely toy idea is driving an HDMI monitor with bit banging.  There was already a hit in the lion kingdom's history.

https://hackaday.com/2019/07/26/hdmi-from-your-arduino/

This was for a music display, but there was nothing cheaper than a raspberry pi in the end.

Turnkey solutions which generate HDMI from microcontrollers are all large, expensive boards with FPGA's.  Historically, it was possible to drive composite monitors from a GPIO so it's only fair to believe the HDMI standard left a similarly simple back door for driving HDMI monitors.  

The practical application for this is the availability of displays which cost a lot less than hobbyist SPI panels, but only take HDMI.  The HDMI signal would be very low res.  The 4k monitor only goes down to 640x480.  It would need to push 18.4 megapixels per second.  NTSC systems only needed to push 4.6 megapixels per second for 320x240.  A fast ARM might do it.

It would have to be some realtime analog application that a raspberry pi couldn't do.  Then there would be a benefit in combining the display & application on a single chip by creating HDMI in software.  An oscilloscope, audio display, or software defined radio comes to mind.

The killer application is driving multiple 1366x768 displays from a single raspberry pi.  The only way to do it is multiple slow raspberry pi's.  The fastest pi's only support 1920x1080.  1366x768 is too high to bit bang.  There might be a way to drive multiple 1366x768 with an FPGA.

The Novatek NT68676 takes analog SVGA as well as HDMI, but not composite.  Since the cheapest display is a replacement LCD panel + NT68676, it might be easier to bit bang SVGA for 640x480.