If one of the creatures it returned would leave the battlefield, it'll be exiled instead
Then theres also cantrips (Ponder) and cards that can be drawn from ETB effects of various spells (Spirited Companion)
Some of the most valuable cards include Field Blower, Sylveon GX, and Mallow, with Sylveon GX (Rainbow Secret Rare) holding the top spot
However, we do know Ursulas Return will include Bruno and the rest of the Madrigal family from Encanto
Proxies featuring these arts (either hand drawn or printed out) are also allowed, as are Blacker Lotus and Chaos Confetti (as proxies for Black Lotus and Chaos Orb)
You can have more than one background card as part of your 99, but you can only have one background as a second commander