@pedronis - this is a good question - we also have a similar request over in Yaru dark theme toggle review request - in both cases we have software that wishes to make itself available (as a plugin/extension etc) to existing software installed on the host system - so classic is the only possible solution we have to this problem - and I agree that this does not really fit with the snap model. In particular the way these applications on the host system support plugins as files within a well-known location makes this hard to support with snaps.
The most similar thing that I am aware of which we have tried to support like this is GNOME Shell Search Providers - but in this case we are exposing a D-Bus interface and allowing snaps to use that interface to register themselves as providers - so for the case of these plugin type applications we would either need to provide something similar (ie a well known D-Bus or other API which snaps could plug and then use to register themselves - and this might in the background create the appropriate plugin file on disk etc for the host application). However I can see this having it’s own issues around API stability etc between the host application and the plugins which we then create via this proxy interface.
So in summary I tend to agree we can’t really support this use-case well with snaps as it currently stands, however even given this I am not sure that classic confinement is an appropriate solution due to the large amount of unexpected authority which this grants.