I need to get down to 8 rare artifacts/colourless, and I'm currently at 10. Those break up as:

Pseudo-creatures:

• Celestial Totem - only usable in a white deck. Best in a white deck with several cheap enchantments and ideally some cheap creatures too. Huge Melvin appeal as a reference to the Weatherseed Totem cycle (see Weatherseed Treefolk) and Celestial Ancient. Marginal plot relevance.
• Draconequus Doll - The only actual creature in the bunch, and not much of a creature really, but fun get-things-moving chaos card that works nicely with scry effects. Might contribute to ground stalls maybe?
• Salvaged Wreck - Plays the role of the big stompy rare artifact creature. Seems more plot-relevant than it is? As in it only actually appears in two shorts, but the band The Rainbooms are massively important.

Amulets:

• Amulet of Magic and Amulet of Empathy - part of a 7-card cycle; must stay.

Others:

• Reserve Rack - A nifty tweak on Masterwork of Ingenuity that works with the mill theme. Semi-works with the legendary equipment.
• Strionic Amplifier - Solid interesting rare for a moderately Song-heavy deck. References a fun gizmo from the climax to Rainbow Rocks.
• Twilight’s Blueprints - Powerful scry-type effect and flexible build-around for an enchantment-heavy deck, or an artifact-heavy deck, or a creature-heavy deck.
• Faculty Lot - I've cut the rest of the cards that care about Vehicles, but this one I think stands alone nicely. It's a great reference to a running joke in Forgotten Friendship, a nice commentary on the difference between the human and pony worlds, a powerful include in any red or white deck with any Vehicles.
• Crystal Cavern - This is a straight upgrade to Crystal Quarry which is no bad thing. It enables splashing anything up to and including Sunset Seraph once you get to six lands. Plot-wise it's a distinctly relevant location in the fourth film.

On the chopping block are the Totem, the Doll, the Reserve Rack, the Amp, the Blueprints and the Cavern. Two of them must go.

­Celestial Totem:

• Flavour/relevance: 1/5. An item that was mentioned for a few seconds in film 4.
• Gameplay/Fun: 3/5. Mana rock early, flying beater midgame, pump engine lategame.
• Fit with set themes: 3/5 - the set has a lot of enchantments, not many tokens
• Uniqueness: 3/5 - plays similarly to Vehicles, but there are only 2 of those left
  Total: 10

­Draconequus Doll:

• Flavour/relevance: 3/5. It's a half-second gag in an Equestria Girls short, but a great reference to the Spirit of Chaos from Equestria.
• Gameplay/Fun: 3/5. Chaos, as long as the opponent attacks with a ground creature.
• Fit with set themes: 1/5. Scry.
• Uniqueness: 4/5. It's the only actual creature and the only rare with cost >4 mana.
  Total: 11

­Reserve Rack:

• Flavour/relevance: 1/5. A throwaway line and visual at the end of one ending of a multi-choice short.
• Gameplay/Fun: 2/5. Only does anything if an Equipment has been seen, and not if it's a Legendary Equipment still on your side of the table.
• Fit with set themes: 3/5. Legendary Equipment are a thing.
• Uniqueness: 3/5. It's a Body Double tweak on Masterwork of Ingenuity. By definition it'll not do anything more unique than whatever other cards are around.
  Total: 9

­Strionic Amplifier:

• Flavour/relevance: 4/5. A pretty iconic item from perhaps the most popular film.
• Gameplay/Fun: 4/5. Only does anything if you have Songs, but players like doubling stuff.
• Fit with set themes: 4/5. Songs are the set's biggest theme.
• Uniqueness: 2/5. The set does have a bunch of -cost artifacts and cards in general, and it's unfortunately similar to PostCrush, Echoing Duet.
  Total: 14

­Twilight’s Blueprints:

• Flavour/relevance: 2/5
• Gameplay/Fun: 4/5. At worst it's an expensive scry each turn, and it'll usually draw you a couple of cards as the game goes on.
• Fit with set themes: 3/5. The set encourages some enchantment decks, or artifact decks, but usually you'll name creature or land.
• Uniqueness: 2/5. It's a 3-cost scry-sometimes-draw effect.
  Total: 11

­Crystal Cavern:

• Flavour/relevance: 3/5. An iconic setting from my favourite of the long films.
• Gameplay/Fun: 3/5. It just fixes mana, and only late-game.
• Fit with set themes: 2/5. It helps you cast most of your gold cards I guess.
• Uniqueness: 3/5. It's the only land that makes coloured mana above common.
  Total: 11

Looks like the Totem and the Rack bite it.