What the hell are they going to create that's better than 1000 other minigames I can get on my phone?
This largely depends on the implementation and social aspects within said game. Housing for example, as implemented by some games, can be a very social thing for players to hang out and chat, or it can be a dead system no one wants to use, because no one sees the point in it. Which is probably the fear the WoW devs have with the system. And to be honest, in the context of their game housing could go horribly wrong, as evident by the abandoned garrisons.
Another interesting idea is "playing an instrument". Sure, you could play one in real life, or just play an MP3 or MIDI file. But in the social context of MMO games it somehow is recontextualized into something social.
Does it work in all types of games? No.
Should the developers try new things in their game and see what sticks? Yes.
Basically, more activities? Yes. That involve actually playing the game. And, really, what the game needs is some medium (with hard mode) group dungeons at endgame. Some group content that's fun and challenging.
So you want content for player power progression, apart from the very hard raids? At this point, as a general rule, you, and everyone else, should be careful what they wish for.
One finger on the monkey paw curls.
You get group content for player power progression (known as "gear") inspired by the market leader: You get hard dungeons, and then increasingly difficult "mythic" dungeons. But the increasing difficulty is mostly gimmicks, more damage/HP on the mobs, and a shorter and shorter timer to finish the dungeon. You also need "keystones" to unlock those difficulty levels, which you have to build up by level, and those keystones deplete in level when you fail a dungeon timer.
In turn, the dungeons are designed for specific paths and playstyles, and players are discouraged from innovating, or else it either gets patched or the players get the banhammer. While the first iteration (let's call it "Legion" here) was fun alright, they are streamlined, the edges sanded off, have more gimmicks, which are less and less fun with time. It looks like design decisions were made with Excel spreadsheet formulas. Over time they degenerate into chores you have to exactly play out according to the meta, or else you get flamed to oblivion by your peers. In this high-stakes environment players get increasingly annoyed by each other, especially at failure, and get toxic.
The developers on the other hand don't want to be seen handing out "high level gear" that was previously seen as status symbol for players who cleared those "very hard raids" to players who slowly grind their way up on a difficulty scale. The difference is seen as a high jump bar and a ladder.
If you have any ideas how to solve these issues, especially without neutering the inherent challenges of those different playstyles, while also not disparaging players from trying the raids--which are seen as something akin to the "crown jewels" of the game--then I think there are some people in Irvine, California, who would certainly like to know and copy your homework.
I only log in to this game to grind gold, and occasionally level up a new class. I want to have more reason to log in, something to look forward to, and I just don't see a few random minigames doing that.
You want power progression (aka "vertical") and want the devs to exclusively cater to your playstyle, and don't want them to develop other forms of progression (aka "horizontal") with different rewards (e.g. cosmetic).
At the same time you say it's "Korean MMO 101", because everything else, especially cosmetic items, are put on the cash shop. Here's an idea: Use FOMO. Put cosmetic things as a reward on time-limited horizontal progression systems, in something like "events". After the event has concluded you can sell the no longer available rewards on the cash shop.
