I will sure take a look at writing a plug-in some time (don’t expect it in another few years) but this is a feature I’d like to see in Krita’s core. If you do this with a plug-in all the brushes will depend on it making it impossible to share them without and maybe even have other issues. I don’t even know if it’s possible with the current API.
However if someone can point me in the right direction in the python API, I may consider doing it if possible in my rare free time (however I’d rather paint then ;3)