Heat Shimmer is great, as not only can you copy a creature for their enter the battlefield ability, but it can also combo with Dualcaster Mage for an alternate win condition
This has a passable baseline as a three mana 2/2 that can tap for Green, and most Green decks in this format will have enough Elves for there to be additional upside
This style of flicker deck usually favors a commander who can enable flickering itself, and you can review my selection of the best commanders for enabling flicker decks if that is your preferred style of gameplay
Maybe it wont end up fitting in U/W Spirits, but I certainly want to try it
You already have Maurice - World-Famous Inventor himself plus Judy Hopps and Pawpsicle and Hiram Flaversham
I WANT to pay attention, I do, but some people's brains don't love that sort of prolonged concentration and yearn for outside stimulus