I don't readily know of any format that would need more than 400 cards in the main deck, and a 100 card limit on the sideboard would still let people import whole sealed pools as decks.
I frequently run into players in the Modern Open queue running decks between 600-900 cards. Is it
needed? No. But the game engine currently seems to be able to handle
combined deck sizes of up to 1600 cards fairly well (which is why I suggested an 800 card max). Even a 700 card max would allow a total combined card count of 1400, which is currently the (untenable) single deck max.
As mentioned elsewhere, I have a couple Buddies I've played 1400 v. 1400 for years with no problems, until the game engine was moved to Daybreak servers. Currently, one of them insists on running a deck close to 1200 cards, which means I have to keep mine in the 400-500 range to minimize the crashes. The "format" you mentioned we loosely call "Complete Cube." I.e., take a complete cube (usually 540-560 cards), add enough land to support it, shuffle up and play. Yes, this essentially gives everyone a Prismatic Singleton deck (depending on the Cube, and the added lands). And yes, you have to build your strategy on the fly instead of in the lab. Granted it's not for everyone (Spikes would abhor it), but don't knock it until you've tried it. It "
is not about well-oiled machines. It's about the fun of the same deck playing out a different way each time, seeing bizarre combinations of cards that would normally never see play, and throwing all your favorite cards into one superdeck..." -Doug Beyer, Magic Senior Creative Designer, [writing about the formerly-supported Prismatic format].
True not many people play it, but the game engine appears to be able to handle it if the deck size is held to 800 or less.