The way I’d like it to be is basically a number of neighborhoods being created around the world (different locations for different themes, like Mulgore with Tauren structures, Ashenvale with night elven ones etc). There, Blizzard could approach it in their typical way of making buildings that cannot be entered with the exception of one - the housing entrance.
This way from a roleplaying perspective, you can have people rp around an area where they live and be neighbors with others without Blizzard needing to create a specific house for each specific person in the open world. The way I think this could work is that you have these culture based houses within the different parts of the world, allowing the players to not just have a human based house when they are playing something else just to give an example.
Now while the outside of the house would not be as customizable because of that (they could technically make these neighborhoods be phased like the garrisons were, but that does cut off a big part of the world rp possibility), however at the same time the neighborhoods would still fit inside the world and all that. Wouldn’t be like having a Ardenweald based house somewhere among Forsaken based ones inside the Plaguelands etc.
At the same time I do think that there should be an area that involves a more… free approach. This could be best achieved by revamping at least the capital cities of the factions (SW and ORG) and creating some sort of housing district within? Again, this could create something on an immersion breaker by having completely different themes next to each other, but I feel like if there should be a place for this, the capitals are the answer (after all, we’ve got multiple races and their cultures as members of the factions, thus even lore-wise the capitals would be the best places to have this culture blend area).
Now as for entering the house itself… I think people should have the option to themselves select from a number of categories (overlaps possible of course) of what kinds of people may enter their house. Friends, guild members, specific people on a whitelist, party members… And on top of that an option to decide whether the people who have the permission to enter your house can do so only when you are inside, or even when you are not (this could be ideal especially if some people want to roleplay living with someone - the roommate shouldn’t have to be there for them to be able to enter).
Customization? The more, the better I say! Now I do not have much experience regarding housing in other MMOs (bar a private server that someone did already mention here), so sadly I am not all that aware of the possible limitations this might bring. But the more options we have, both with things we want to place somewhere, where we want to place them, their scale rotation… the more, the better.
But yeah, just a few morning thoughts about this! In the end, I am very happy they are asking for feedback and hope they will indeed listen. Now I do not want to reduce the voices of the others but I do think that they should listen to the voices of roleplayers the most mainly because I feel like they are the part of the community that would be able to get the most out of the housing system.