"Imagine being in Minecraft, then walking your avatar through a portal that takes you into Roblox. Or more likely just hitting a button and being there."
Microsoft has actually been experimenting with this technology for a while - their implementation is quite sophisticated, and it's shipping in Windows now. You can actually try it out today: open Minecraft, then press the World Switcher hotkey combination (press the Tab key while holding Alt), press the Windows key, type "Roblox", and press Enter. If done correctly, your avatar will now be in Roblox!
It's pretty groundbreaking stuff, to be honest. Under the hood, the old universe is dynamically unloaded (possibly swapped out to disk) while the new one loads in. And, even better, if you press the World Switcher hotkeys again, you'll be right back in Minecraft - exactly where you left off.
...
Of course, I'm sure you meant that you'd somehow share information between Minecraft and Roblox- I'm sure you'd agree that arbitrary in-game items are unlikely to be transferable, but at least avatar appearance, surely? Well, hmmm, that'd make both games look worse, with awful art-style clash. Well, I guess you could store a few parameters and transfer those, and let each game interpret them to render an avatar that fits in the game's style?
... Ah, I've just made Miis again.
Okay, well, hm, if transferring avatar appearance isn't especially interesting, what about stuff that doesn't directly correlate to anything in-game? For example, the social graph, friends list, communications. Imagine being able to see what worlds your friends are in, talk to them while in your current one, and join their worlds with just a cli-
... oh, that's just Steam and Discord.
So... what's new, here?