It would be really nice for Home Assistant’s overall organization if we could attach an entity to an existing device and/or create internal devices.
For example, attach helper entities that were created to help with automation or for statistics related to entities that already exist on a device.
Or create an internal device to group several helper entities related to a specific use.
I think it could go like this:
Add the ability to create devices in current Helpers.
Add the ability to include a device in the configuration of entities that are not linked to one. On the frontend it can be similar to the area configuration.
Devices are not really deeply integrated into HA and you should somewhat ignore them.
HA works on entities and there are very few device specific functions, and certainly none for custom devices, so grouping them in devices will make no real difference.
I think devices is mostly for integrations to be able to handle a group of entities, like deleting and creating on connection changes and if you have custom entities, then they might not be handled well by a deletion and later recreation.