Thanks for the roadmap update! 
And nice to see that the fridge example (Roadmap 2024 Mid-year Update: A home-approved smart home, peace of mind, and more! - #29 by Lakini) survived through the various occasions that it popped up in.
One thing I find confusing about this post though:
“we don’t always know that those entities together form, say, a fridge”.
I’m not sure which type of problem you are planning to address.
a) a Samsung smart fridge that already today shows up as one device has a number of entities, but Home Assistant on a higher level doesn’t know the concept of a fridge and can therefore e.g. not propose good automations?
b) a “dumb” fridge was made smart by combining the entities of OTHER devices (magnet sensors, temp sensors, power measure plugs, …), but HA doesn’t allow to easily combine those entities into a new device?
c) both a and b?
If it’s option b (allowing users to combine all kinds of entities to “new” devices), then
“we also need a centralized, structured place to store and share device knowledge”
would still be a great initiative in itself, but wouldn’t solve the problem. One dumb-turned-smart fridge might rely on a self-built esphome node, another one might be based on the combination of various zigbee devices from different companies. Or are you planning to not just have a device database that allows people to store information about concrete models (“This is the Samsung SuperSmartFreeze 2000, …”), but one that allows the community to enhance HA’s overall understanding of household items by providing templates on a higher level (“a fridge is usually used in the kitchen, in a food/cooking context. it might have multiple door sensors, temperature sensors, and a thermostat setting, …”)?
Another thing that is halfway related to the topic:
“What’s the temperature in the kitchen?” shouldn’t return the freezer’s internal temperature.”
This is coming from an exclusion side so to say (=> what NOT to take into account for a question). What would be great to implement instead or as an extension: the ability to define the primary/leading source per sensor metric, per area or context.
Example: my HA has 3 sensors that provide an outdoor temperature. A zigbee sensor to the north side, an esphome node on the balcony to the south, and a weather forecast service temperature. At the moment there is no clean way to define which one the leading should be for the voice assistant to answer a question about the outdoor temperature. I could of course create a template sensor named “outdoor temperature” that takes the values from the other 3 with a logic of my liking. But that’s not necessarily beginner-friendly
There is more thoughts around this in my head. As mentioned in other contexts before: the moment you got it sorted how you would like to organise non-NC-productmanagement-input from the community (from people like me who cannot code well enough to contribute feature-wise ), just let me know, happy to contribute!