There is a lot of discussion about whether the changes should be stored in the cache or on the disk. I propose a compromise - that is, both - on disk and individually.
Example:
Option to make default sidebar items and order, independent of cache (saved on disk). This view will be default, but user can change it for every browser / device and these changes will be saved in cache.
In configuration.yaml we put code for default sidebar view. Every user can rearange this view (long click on HA logo). When he clear cache view will back to default view created in configuration.yaml
One feature that is talked about a bit in the WTHs are whether the GUI setup should be store locally on the device or server-side.
Locally means every device have its own GUI setup and settings are not copied from one device to another.
Server-side can both mean that GUI setup is per device or it can mean it is per user or it can mean it is global. The way it works depend on how devs implement it and how the can support it.
The other feature is whether there should be a default user-customizable GUI setup.
This is not depending on the other feature.