Retailer and prerelease promo cards tied to new launches tend to spike for short, profitable windows, and reverse-holo complete your set singles do steady work for collectors filling binders
I know this comment is 3+ years old now, but the description in the 1.2 version makes me think the Gengar thing IS intentional -- but yeah, it could be more clear

Synthoids Aang and La, Ocean's Fury Aang's Journey Aang, Destined Savior Aang, Master of Elements Aardwolf's Advantage Abandon Reason Abandon the Post Abandoned Air Temple Abandoned Campground Abandoned Outpost Abandoned Sarcophagus Abbot of Keral Keep Aberrant Aberrant Manawurm Abian, Luvion Usurper (playtest) Abolisher of Bloodlines Abomination, Irradiated Brute Abomination, Terrifying Titan Abomination, World Ravager Aboroth About Face Abrade Abraded Bluffs Absorbing Man and Titania Abstergo Entertainment Abstract Iguanart Abstruse Archaic Abundance Abundant Countryside Abundant Growth Abundant Harvest Abundant Maw Abzan Banner Abzan Beastmaster Abzan Kin-Guard Abzan Monument Academic Dispute Academy Manufactor Academy Raider Academy Ruins Academy at Tolaria West Accelerate Accelerated Evolution Accelerated Mutation Acceptable Losses Access Tunnel Accessories to Murder Accomplished Alchemist Accomplished Automaton Accorder's Shield Accursed Duneyard Ace's Baseball Bat Ace, Fearless Rebel Acid Web Spider Acidic Dagger Acidic Slime Acidic Slime Acidic Soil Acolyte Hybrid Acolyte of Bahamut Acolyte of the Inferno Acorn Catapult Acorn Harvest

You can see all translations for Ninja Spinner here and all its secret rares here
You'd have plenty of duplicates of other Rares and ~35 total Mythics but finding those last Rares is always tricky
Each carries a certain weight, which well discuss to help you estimate the appropriate number of lands for your deck