There are some players that are Achievement Hunters. They don't care about the rest of the game; only increasing the Achievement Points number. Websites like WoWhead build systems that tell you which achievement should be next, based on your progress.
Players become emotionally invested in their achievements, and come back for more. They don't want to unsub because they're proud of their progress.
There are some players that are Transmog Collectors. They don't care about raiding or dungeons or Battle pets; only completing the Wardrobe Collection numbers. Addons are created to help them hunt for what they're missing, and economies are built on selling them rare transmogs.
Players become emotionally invested in their collection, and come back for more. They don't want to unsub because they're proud of their progress.
Now imagine how Player Housing would impact the game.
You would create an entirely new player base that would play WoW solely for building up their player housing, instanced like the Darkmoon Island. The technology is there! Just combine it with the Garrison building technology and it's good to go!
Seriously, I love my Garrison. I still go back to see if any new, interesting NPCs are available.
Take the idea, combine it with Darkmoon Island instance tech, let players build the foundation and let them use any racial/faction thematic appearance they want (contingent on unlocking the theme via levelling alts/Allied races), add roaming pets and mounts and npcs you've collected, plus a constantly increasing list of furnishings, trophies, achievement hallmarks, and outdoor features... and holy shit I would spend years building it up and collecting it all. At my own pace.
Even if I was happy and thought it "finished", I'd still drop by just to spend time with the NPC's/mounts there.
Just as Battle Pets adds all new pets to farm from old raid instances again and again, every expansion could see all new furnishings added to your Player Housing, with the latest expansion introducing new collectible furnishing, trophies, outside features, and more every new tier. The garrison had limited racial themes, and seasonal furnishings. Players wanted that... only ramped up 1,000%!
Every single expansion, Blizzard's PH team could add new thematic features to the player housing for people to farm, hunt, and collect, all found in every feature WoW has to offer: raids, dungeons, PvP, pet battling, World Quests, Levelling quests, achievements... just like people do now for transmog and achievements they just HAVE TO HAVE for no other reason than they don't have it yet, and leaving gaps in their Player Housing Features tab triggers them massively. And if they don't want to have to run difficult current content to get them, they can just wait an expansion or two until they can solo the content. That means YEARS of subscription payments.
People say that cities would be deserted if you added housing, but that would be easily prevented by two simple things already in-game:
- Active NPCs. Blizzard added NPCs running in and out of the portal room to give the appearance of high traffic. Say what you will, but it works. Whenever I port in it's always busy, and I honestly don't notice who is and isn't real. Just add more NPCs to the main cities, and you have bustling hubs helped along by sharding tech.
- No Essentials. DO NOT put in game-affecting features; NO Auction House. NO resource farming (mines, herb gardens, etc). NO bank. Not one thing that you can find in cities. Player Housing should be like IRL housing; comfort and appearance only. You go there to relax and admire what you've built. Players would NOT just linger in their Player Housing because like the real world, they need to leave to actually do essential stuff.
I've been a high-end raider, and it was fun... but I outgrew it.
I've been a high-end PvP'er, and it was fun... but I outgrew it.
But the one thing I have consistantly done year after year after year, without fail, is get those old Achievement Points and get those old transmogs. Because the beauty of those two systems is that you can do them hardcore AND casual; spending all day/every day doing cutting edge content or just logging in a few minutes every now and then to buy cheap uncollected transmogs/pets off the AH or maybe farm that elusive mount or run an old dungeon/raid. Just slap a XXX/YYY tally number on it, and you got 'em hooked. The cost is sunk!
Players would look at their Player Housing and be utterly emotionally invested in it. They would see the AGES they spent collecting housing features, furnishings, trophy collections, and wandering NPCs (yes, like garrisons) by farming old content, levelling old zones, raiding, running dungeons, doing PvP, Pet battling, doing old zone quests, levelling alts, and so much more WoW content to unlock and furnish their player housing and feel so damn happy, with a sense of Pride and Accomplishment™, just like I did whenever I went into my Legion Class Order Hall and saw the rows of Tier Sets I collected. Once I realized this "Hall of Fame" feature existed, I had to collect them all!
And every expansion, you would see more furnishings and features added! Both obtained doing new current content AND more added into old content, meaning the players would have to go back and run them all over again (just like collecting the music for the garrison Jukebox, or farming old raid instances because they added more pets to the bosses. Was I happy they did that? NO. Did I get those pets anyway? YOU'RE GODDAMN RIGHT, I DID.)
I honestly believe Blizzard had no idea how successful transmog was going to be. To be honest, I think every single success Blizzard had was an accident. They tried something, and were completely blindsided by how fast it was adopted (like WoW itself). That's why they keep adding new systems again and again every single expansion; they throw new things at the wall and see if they stick, because the ones that did (transmog, achievements, world quests, mythic +, etc) were UNBELIEVABLY HUGE successes, so they don't mind if 9 other systems crash and burn and are hated from the moment they hit PTR; it's that one that succeeds that makes it all worth it.
Player Housing could be huge. It could be revolutionary. It could create an entirely new player base that logs every day, every month, year after year, endlessly subscribing and filling Blizzard's coffers for no other reason than to keep building and renovating their villa/fortress. And with Darkmoon Island technology, it's growth could be limitless. And by sticking to the hard line of NO ESSENTIAL SERVICES (ie. bank, AH, barber, professions, etc), players would not just stay in there forever, never leaving and turning major hubs into ghost towns. Housing would be personal, cosmetic only, filled with your pets, mounts, and NPCs you could collect and chose from that would interact with you... and you have a feature winner.
EDIT: Try to ignore the usual negativity of the 10yr MMO accounts. Remember: they're flagged Red for a reason!