One variant format that has risen to prominence in recent years is Cube, a type of Limited with a predetermined, varied or themed card pool. "Powered" Cubes, especially, in which every card is meant to have a massive power level, are the favorites of custom Magic card designers who crave complex and even complicated interactions between cards.
Tumblr user bestquestion appears to be such a player, asking Mark Rosewater on his Tumblr question-and-answer blog, "I love that you guys do all the supplemental products, but I feel like there hasn't been one targeted at my demographic. "Multiplayers" get Commander and "Super-Casual" get Planechase (also UN-sets). I'm a Cube player that wants "super-complexity" Melvin cards. A Time-Spiral lover. I mean Modern Masters was fun but in the end, it had absolutely no new cards (and so it really 'did' absolutely nothing for my demographic). Are there any supplemental products coming in the next decade for me?"
Rosewater's answer was considered:
We do make some every year at higher rarities in normal expansions.
Is there enough of you to warrant a supplemental product? Maybe. It’s something we’re exploring.
The mere hint of an all-new-cards product for Cube building is a bombshell to the segment of Magic players who enjoys the Cube format. While this question didn't get a massive response, most of the responders were quite passionate, saying things like "YES. I love Cube and it’d be nice to see some more complex cards. Time Spiral was awesome and, though I understand the need for it, NWO makes me sad sometimes," and "Time Spiral was my first and favorite set, and it’s probably because of that that I’m a huge Melvin. I would love the hell out of a high complexity supplemental product."
Time Spiral block keeps coming up in this discussion - although Mark Rosewater himself loves it, it was the high point of "complexity creep" in Magic and a barrier to entry for most would-be new players, as fully appreciating it involved understanding both the many keywords that were used in it and the references to 13 years of Magic's past. Ultimately, there seem to be enough passionate fans of Time Spiral-esque design that they'll get a "consolation prize" - a supplemental product of their own someday.