Uncommon artifacts/colourless are pretty tight. I need to get down to 10, and I'm currently at 11. But those break up as:

• The cycle of 5 legendary Amulets (WUBRG) for Sunset Shimmer and the Equipment theme
• Vending Machine for the proliferate theme; I finally cut the Glamping Van
• Camera Drone for the heroic theme
• Decorated Detector is a really nice Crystal Ball that also enables Nerd Chic, Piecake Eating and Throw the Horns
• Steward of the Unnatural and Animated Fossil are decent-sized creatures, especially in this set that's full of 2/2s and smaller.

There are only 3 creatures on that list, so I think they all have to stay; it's one of the noncreatures that has to go, and I think it's the Van or the Detector :(

Already cut:

• Saturated Media is a wacky card for a deck with a few Songs and a few non-Song enchantments. The set does have a load of non-Song enchantments, but not every deck will.
• Cinnamon Crowbar - total unblockability isn't very interactive
• Necrobeam Staff - not enough gold cards in the set to enable it, and black already has tons of lifegain
• Danger-Prone Dock - it's a great reference to a fun recurring joke, but doesn't do very much against most decks, and on the occasions it does, it might actually manascrew you
• Rarity’s Workshop - it fits the heroic theme, but doesn't do very much
• Hero’s Cape - it fits the heroic theme. It doesn't do much with sorcery-speed heroic triggers, but instants can be great... though it only grants +1 toughness inherently. Also it's hard to find specific artwork: "superhero capes" were discussed in some amusing dialogue, but none of their outfits actually have capes.