Well, Arcana, you've more or less described the TORG MMORPG that I've had rattling around in my brain for the last fifteen years.
The problem with the multi-game universe idea is not so much that variations have never been tried. It's that the people who try it are always indie developers who attract a bunch of participating dev teams that can generally only loosely be called "developers" at all. The problem is that nobody wants to play a bunch of no-name games developed by amateurs. People expect AAA games from a MMORPG experience, and for the reasons covered previously, the genre as a whole is scaling way back these days.
I thought that Landmark might be the thing that would crack the genre barrier. The premise of a single world with every player building their own "world" within the main world, where my Space Ranger and your Super Hero and Tim's Enchanter could all adventure together in a multitude of "lands" each with its own rules, theme, and landscape was an amazing idea. It's too bad that it was too grand an idea for SOE to really crack, and their focus on EQ Next meant that Landmark took a back seat. Now that they're struggling just to pay the bills, I don't see that Landmark will ever be more than a dream of what could have been if the hype, and Storybricks, had panned out as advertised.
The Multiverse" platform as built on the idea of a common shared platform that could have made it possible for two or more games to share users between them. I'm not aware that any of the games that were developed on it were ever made complete enough to be playable, let alone cooperative, though. Granted, it's been a long time since I last checked in on them, but their website seems to be broken-down at the moment which suggests that Multiverse has fallen apart and the promise of a common platform along with it.
I recently got interested in an online fiction platform by Failbetter Games, the makers of Fallen London and Sunless Sea. Sadly their platform, Storynexus.com, is on life support as well. They idea of a shared platform of interactive fiction games that could, if they desired, transfer achievements between each other, also failed to pan out as a commercial venture. They keep Fallen London going because it is self-supporting but they don't even bother to support Storynexus any more.
The idea is never going to take off without a big name, big budget AAA producer deliberately supporting it from day one, *AND* managing to make their first game or two in the platform a resounding success. Especially since they'd need to support each other and not cannibalize each other the way that CoH and CoV did, and the way that EQ and EQ2 frequently did.
Given how MMORPG's are going, I don't think it will ever happen, explicitly.
Implicitly, it can happen naturally as an outgrowth of a game's aging process. I read an article recently in which a Blizzard developer talked at length about the challenges of developing for a "community" that was really an aggregation of many small communities focused on one or two aspects of the game: PvP, pet battles, transmog collection, market flipping, raiding, achievment grinding, socializing, etc..., etc... His contention was that with each expansion they have to design it for twenty different versions of the "WoW audience" because the "WoW audience" doesn't actually exist as a monolithic entity.
WoW may not exactly be HyperWorld but I can certainly see where the guy was coming from just from the way I play it myself (or don't play - it might be time to cancel my sub again until Legion drops).
I've personally been waiting a long, long time for TORG World or GURPS World or Rifts World - It's just never going to happen because to make HyperWorld a success you first have to make five sub-worlds each be an individual success. even before WoW, hitting a single success was pretty much lightning in a bottle. These days, I don't see how a virtual world game is even going to get launched. Technology and changing expectations have led people to realize that you don't have to have a Second Life in order to escape online for a bit of entertainment in something that feels like a virtual world even if it's not really a persistent "world" like a MMORPG.
Look at CoV. Strictly speaking, it was a failure as a stand-alone game. The joining of the two games was just an acknowledgement that the people playing both games were the same people.
I don't know what that means for online gaming communities. I don't see the same sort of communities forming around Hearthstone or League of Legends.
Maybe I should look in on The Secret World. I haven't dropped in there in a long time and it would be useful to see what their community is like these days.
***EDIT***
Just as an addendum - I would suggest that Free Realms and Superhero Squad are both pretty much just what you described for HyperWorld. You login to the overarching "social" world and then branch off to the other "worlds" as you wish either on your own or with your friends. It's probably not too encouraging that both of those games are (or were, in the case of Free Realms) kid's games. It seems like the concept of a "world" built of many different and separate activities is treated by the game dev community as some kind of acknowledgement of developing for short attention spans rather than a way to offer a kind of variety to an adult customer.