The previous option was to provide small PCB satellites which are programmable in BASIC.
This didn´t give a good feedback from the community.
But I received several questions regarding using your own hardware - which of course is the more exciting option.
This seemed impossible as the coordination of so many participants - including flight qualification testing etc. would be too hard to accomplish.
But I came up with a plan to make it possible anyway - the only thing is that your hardware can´t be deployed independently - it must stay within the satellite.
The good thing: You will be safe in a satellite bus, provided with 3.3V and radio - you don´t need to care about regulations and paperwork. You also don´t need to care about other modules. You will have an UART interface and can send your data down to earth easily.
Are you familiar with SSTV? It´s a way to send a color picture via a low bandwidth radio within less than two minutes.
My plan is to give every participant a dedicated area in that picture to write your data in. It can be either a graphics or a written text.
You are free to write on the canvas - and receive it e.g. via an RTL-SDR with your laptop and a 10$ antenna.
The orbit would be probably a polar orbit which would allow you to receive the signal anywhere on earth 1-2 times a day.
Communicating with other modules would be also interesting - for example using inertial data from another module or getting a camera picture for analysis or using a "reaction wheel" for turning the satellite around and and and...
Your own hardware will get a small window in the satellite outer shell to the endless space making it possible to use optical and other kind of sensors.
There will be also a simulator so you can start developing. All you need is to connect an UART to the PC. The simulator wil behave like the carrier satellite.