Regarding this logic, i noticed a small problem. If a metainfo plugin supports multiple id's and no actual plugin for those id's exist than the field name is not retrieved.

Example:
series_identifier_fields = [p.instance.series_identifier for p in plugin.get_plugins(group='series_metainfo')]
This will never get the tvrage_id, so this field still needs to-be hard-coded.

So maybe there should be a extra property or additional list, tuple return of all possible supported id fields. So that we can always get a complete list of all id fields for each category per plugin.

Maybe like this?

    @property
    def series_identifier(self):
        """Returns the plugin main identifier type"""
        return ('tvmaze_id', 'tvrage_id', 'tvdb_id')
or
        return ('tvmaze_id', ['tvrage_id', 'tvdb_id'])
or
        return ['tvmaze_id', 'tvrage_id', 'tvdb_id'])  # index 0 is main id?

or

    @property
    def series_identifier_supported(self):
        """Returns all supported series identifier fields"""
        return ['tvmaze_id', 'tvrage_id', 'tvdb_id']

What metainfo plugin has multiple ids? And tvrage isn't returned because we don't have a tvrage plugin anymore

tvmaze has support for: tvmaze_id, tvdb_id, tvrage_id
tvdb has tvdb_id and fills (imdb_id, zap2it_id) (api supports also more)
tmdb has tmdb_id, imdb (the actual api also supports tvdb, tvrage for series)
trakt has trakt_id, tmdb_id, imdb_id, tvdb_id (api supports also tvrage_id)

So in most cases the remote api's actually support multiple id lookups and also return multiple id's, even if not currently implemented in the plugins.

In my case i wanted to get all possible series id fields that can be used to identify a series and as noted i still have to hardcode tvrage_id.

The idea of this identifier is to have one main identifier per metainfo plugin type which can then be used in various places (like validate allowed values for examples). While obviously the vast majority of plugins return multiple identifiers per successful lookup, it's not relevant for this usage.
This is also why tvrage id is not there and should definitely not be hard-coded in.

I understand this and by hardcode i mean you need to-do something like this atm.

_series_id_fields = [p.instance.series_identifier for p in plugin.get_plugins(group='series_metainfo')] + ['tvrage_id']

I simply thought that adding the ability to get a list of all supported id fields per plugin might be a good enhancement. So you could search for a meta plugin based on a given id field. Lets say a entry has a tvrage_id and now you want to know what meta lookup plugins could be used for a lookup.

The whole point of this feature was to dynamically add support for identifiers via our existing lookup plugins. Anything else is outside the scope of this