For Time Spiral, Wizards of the Coast tinkered around with a mechanic that was literally nostalgia.
With Magic: The Gathering having a history dating back to the first Bush administration and being as old as the Colorado Rockies, the nostalgia factor easily kicks in for many players. Older players like seeing concepts brought up decades ago come up and younger players enjoy seeing the throwbacks and what the game once was. Like sports, music, video games, and many other things, a strong product can carry over several generations.
Publisher Wizards of the Coast knew this back during Time Spiral, an expansion released in 2006 where nostalgia played a large roll.
Really, it would take a whole video to go into all the ways they tied in older Magic in the expansion, and hey, lucky you, we did that!
But, as much as the expansion went into that, it was originally supposed to go much more into nostalgia. And by that it was going to be made into a whole mechanic, complete with new cards.
Depending on what kind of player you are, it would have either been the best thing ever or would have just completely destroyed the game.
Basically, nostalgia allowed you to have non-standard cards in your deck. As long as the card in question has the nostalgia mechanic, up to four Vintage-legal cards can be held in your deck to be played. If you ever wondered why the ban list was a thing, this would have been exhibit, oh, lets say "Q."
A total of six cards would have been created for the game with the mechanic, with each allowing certain types of vintage cards to go into play. Nostalgic Veteran would have allowed up to four copies of any 2/3 creature legal in Vintage, Nostalgic Wizard allowing the same with wizards, Nostalgic Zombie with zombies, Nostalgic Bolt would have allowed any card with a 1R casting cost, Nostalgic Beast with beasts, and Nostalgic Artifact any artifact card with a casting cost of 6.
Believe it or not, there were problems in playtesting.
Rosewater and company managed to find six subsets of colors and types that would work, but playtesting eventually ruled this out. Another Nostalgia variant was quickly brought up where it was tied closely in with gatekeeping, but once again, it was breaking everything and deckbuilding legality got way too complex. It's hard enough to make an expansion where everything works. And, since Time Spiral was the 40th expansion, try bringing in dozens of those in with all different types playing against each other. It would have been a bigger mess than Disney was during the latter Eisner years.
Nostalgia had to be let go as a mechanic, but it wasn't though Time Spiral didn't have callbacks. It very much did. But you just wouldn't get to play all the cards you wanted to now.
In the end, nostalgia just proved to be too much, but at least everything wasn't broken in the end.
After all, that's Pokémon's department.
