proposal made in Home assistant core feature request, but maybe it is more feasible as a spook feature?
https://github.com/orgs/home-assistant/discussions/1467

Copy of the proposal:

Describe your core improvement

I would like to be capable to specify a customized device class or modify an existing one to better fit my need without having to deeply customize the interface (dashboards).

For example my windows do not look like default HA windows:

but look like the variant icon:

Instead of asking a new device_class every time there is a slight variation, I would like to be able to either modify the definition of the device class in HA or create a custom one: "window-variant" with adequate states and icons.

If I was able to "sacrifice" one of the available device classes to customize it or create a new one from scratch, that would be great!
example with available binary sensors:

This modified, or new, device_class would then be available either in yaml via a customise.yaml or in the GUI via the "display as" drop-down menu in the settings of the entity:

Current limitations

Current device_class offer is already large, but not always perfect, and especially not reflecting many houses opening (windows or cover):

Available device classes for cover:

There is (to my knowledge) no way to customise or create a new device_class in the core.

This could benefit to all types of entities, not only the below examples of binary sensors and covers, but also lights for example.

Technical benefits

Simpler fit to your entities, without the need to create a template for each entity not fully fitting into the current device class offer.

Possibility to create new device class for emerging needs, without need of a core evolution at each time.

For this kind of customization, i do not even think a GUI would be needed, but it would be nice. A new device_classes.yaml file with adequate inclusion would be 100% sufficient, as long as the created device classes are usable in the "display as" GUI as shown above.

A few possible examples:

device_class:
  - binary_sensor:
      - window-variant:
          state:
            'off': 'Closed'
            'on': 'Open'
          icon:
            'off': 'mdi:window-closed-variant'
            'on': 'mdi:window-open-variant'
      - maibox:
          state:
            'off': 'Empty'
            'on': 'Full'
          icon:
            'off': 'mdi:mailbox'
            'on': 'mdi:mailbox-up'
  - cover:
      - double_garage:
          icon:
            'closed': 'mdi:garage-closed-variant'
            'open': 'mdi:garage-open-variant'
            'closing': 'mdi:garage-alert-variant'
            'opening': 'mdi:garage-alert-variant'
  - switch:
      - television:
          icon:
            'off': 'mdi:television-classic-off'
            'on': 'mdi:television-classic'

Additional context

Apparently this was requested many times before on the forum:

https://community.home-assistant.io/t/wth-is-there-no-binary-sensor-custom-device-class/467142
https://community.home-assistant.io/t/create-a-device-class/490957
https://community.home-assistant.io/t/wth-cant-i-specify-on-and-off-icons-for-toggle-helpers/478948/
https://community.home-assistant.io/t/custom-device-classes/368238

It could also serve to customize the state associated colors at the same time as the icon and state.

I do not know if spook is the right answer for this, but it might be...