'Players must leave game in progress as it is and use the cards left in their libraries as decks with which to play a subgame of Magic.'
Following the OSR Megamix line of thoughts, I have accumulated a lot of stuff that could intertwine and play together wonderfully over the last few years. In addition, I'm writing new stuff that will hopefully blow a few minds at the moment and somewhat, all this is falling into place like a puzzle (I know, that's what paranoid people say as well).
Did you play Call of Cthulhu Dreamlands? I did. If you wanted to, you could run two campaign that interacted with each other at the same time, almost with two different characters melt into one, a campaign in the 1920s real world, another in the Dreamlands themselves. I've always loved this idea but found what Chaosium did at this time unsatisfactory since the connections were thin. It felt like the Dreamlands were almost a derail instead of a feed for your character's waking life and that is not what I was looking for.
Yet, wonderful possibilities now exist if you plan to play campaigns on the long run, possibilities that can open your game into subgames and allow to spin-off wildly while keeping in line as long as the main campaign is concerned. Your characters could find the journal that starts Zzarchiv Kowolski's Thulian Echoes in another, totally unrelated dungeon, and they could fall in a pit located elsewhere in this dungeon and enter Dungeonland, where there might as well be a gate to the Demiplane of Ducks (there, now you know the name of my next release, soonish, I swear) - in which, as most of you know, there could be a magical entrance to Geoffrey's Isle of the Unknown (where you also find the Dungeon of the Unknown).
When we think of a campaign, we usually think of an overarching plot, a vilain (mandatory says WotC) and interconnected adventures. When we think of a campaign, we think railroading (or we think sandbox but that's another story) but what if the campaign was just a central hub from which hundreds of subgames could emerge? What if we played a sandbox of sandboxes? Think about the Talisman boardgame extensions, you could start in a classic medfan forest and end up in Deep Space 9 anytime. So why don't we think of campaigns with a Shahrazade Effect?
I'm writing something else at the moment (you won't get any clues on this one here, except for Jeremy Hart's splendid cover illustration) where you can spin into another adventure in a dream, enter another in a musical symphony, open a gate to yet another, fall into one and travel to distant Swords & Planet sandboxes, all this wrapped into a single location in less than a 100 pages. So I'm designing the central nexus hub and whatever happens within but I intend to use many, many other adventures when I run it, including Castle Amber, the Pleasure Prison of the B'thuvian Demon Whore, the Infinite Tower stripped right from Better Than Any Man (again, yes), A Question of Gravity and many more. I intend to play with a full Shahrazad Effect ON. So it's like: you find a castle in your dreams, in which you fall into a pit, in which you find an odd journal, etc.