From the publisher:

Millennium Blades: Set Rotation includes 12 new card sets, giving you half again as much content as the base game, opening up even more variety. Set Rotation is designed with advanced gamers in mind, and gives you additional depth and strategy for experiencing Millennium Blades.

The card sets are:

The Thousand-Pixel Cabinet – Classic arcade homages galore!
Quantum Jump – In space, no one can hear you flip cards.
UUUuurrrrrgh! – The obligatory zombie set. These guys just don’t give up!
Saved by the 90’s – All our favorite pop-culture and TV references from decades past.
Fast – Play your cards furiously with this action-inspired set!
Shonen Leap – These cards are all about the power of friendship.
Deus Rex Machina – Cyberpunk Hackers and Evil Corporations. You’ve never seen these in a card game before!
A Game of Rings – Explore a magical fantasy world where everyone dies suddenly and brutally.
Developmental Issues – Meet the developers of Millennium Blades!
Penny Dreadful – Old-style mystery meets mafia shootouts.
The Ultimate Steel Cook – Deckbuild with a new flavor with these famous chefs!
Ultimate Kaiju Throwdown – Remember kids, only you can destroy Tokyo.

Set Rotation also includes several other new cards:

6 new starter decks, based on card types instead of elements

The Great War (soldiers)
The Academy (mages)
Gods & Myths (myths)
Future Perfect (constructs)
Animal Farm (animals)
Megalopolis (citizens)

Four new playable characters

Pritchard Leftfield – A game designer who dabbles in time travel, and the original creator of Millennium Blades.
The Card Shark
Friendly Alien Lettstrade
Cardinal Baanz

Nine new promo sets (3 bronze, 3 silver, 3 gold)

Four co-op mode decks