create specific firmware online

Hmm, your feedback to userdefined C code sounds like "there is no market"
People with knowledge to do that will already have their own development.
Thats a good point.

So lets go back one step. The idea of predefined C code sounds good to me.
I can imagine some reasons why you did not get lots of contributions

• lack of knowledge how to built
  
• bottleneck for additional functions due to actual restrictions in size
  
• functions are not helpful for majority of users, for example who needs a Schmitt Trigger. On the other hand I would like to have lowpass, highpass, pid control, etc. etc.
  
• restriction to "must run on all boards", for example I've a full color P3 driver, which works on ESP32 only. some guys could be interested
  
• no "how to proceed" for plugins
  
  

Summary, I would like to start a first version of a registry. About 3 years ago I've created a github with my extensions https://github.com/jumjum123/EspruinoExtensions c files.
And never worked on that again :-( Lets give it a try.
I will create an interface to that into my firmware build tool, and remove upload for C files