One feature found in many apps for IoT devices is the idea of grouping rooms/areas together (e.g. zones in the Hue app) but is severally missing from HA.
Personally, what I am suggesting is not necessarily to force a hierarchical approach to areas (which often taken by these apps) but instead to provide a method of grouping particular areas together in which would allow any particular area to be a member of multiple “area groups” (much like how an entity can be part of multiple entity groups).
For example, we could create a group with the obvious groupings areas on a particular floor of a house, which themselves could be grouped into an indoor/outdoor group. But (in the spirit of not needing to take a hierarchical approach) we could also group rooms on different floors together (such as bedrooms which might not necessarily always be on just one floor). This approach should provide people with the flexibility to create a hierarchical structure for their areas but also (if they wanted) the ability to create groups that don’t stick to any particular hierarchical structure.
Adding area groups could address (as what I see as) a slight issue regarding the relationship between automations/devices/entities and areas, which is that currently they can only be assigned to a single area. This forces any entities which functionally span multiple areas (or maybe even physically) to either be placed in one of the room it is part of or just be left orphaned. An example of such a area-spanning entity is an automation I currently have that handles closing the curtains in a couple of the rooms in my house.
Even in providing area groups we could continue to force users to put entities in a “actual” area leaving area groups to simply make it easier to logically work with multiple areas (for example using the bedroom example we can address all bedrooms with a single area group and if someone was to convert a room to a bedroom we can add it without needing to modify any automations), or we can allow users to assign area-spanning entities to area groups (my preferred approach). A small issue does arise from the second option where sometimes we might not want entities in area groups to be affected by service calls targeting just a member of an area group and other times we might do.
To me the obvious solution is to provide a toggle switch in the config section when adding entities/device/etc to an area (group) which selects between these two potential behaviours: 1) Only when group is used 2) Even when any member area is used. We could also provide a similar option in the config for the area group itself to select the default behaviour for when the area group is chosen by an entity.