It can be any rarity, and it has the possibility of being Booster Fun
In a singleton format you have to run a set of cards that all fulfill the same basic function, so instead of running 4 copies of Harmonize, you might run Harmonize, Greater Good, Beast Whisperer, Return of the Wildspeaker, Garruk, Primal Hunter, Shamanic Revelation, Soul's Majesty and Rishkar's Expertise
When you do, sacrifice Fleshwrither, then search your library for another creature with the same total mana value and put it onto the battlefield
Unfortunately, despite being a slam dunk waiting to happen, Nintendo hasnt jumped on that idea either, so that just leaves Ooblets to take the obvious concept instead
The cool thing about this is that with your commander in play, its very nice to combine a wheel with the ability to flashback your cards