1)
Noble Hierarch - It has a huge price tag, and I don't think that it will be game
breaking in standard. This fits well in the Sultai Midrange and Simic Climb decks, but wouldn't make them tier 0.
2)
Manamorphose - Again, this card definitely has a home in standard, this time in Izzet drake decks. This could give them the means to get to tier 1 and doesn't do anything in other shells to be an auto-include in every deck.
3)
Hedron Crab - There is already the pieces of a mill deck in standard, with
Drowned Secrets and
Psychic Corrosion, but this could give it an edge to become competitive playable. It's also surprisingly expensive for how little it sees play. The only downside is it's very Zendikar flavoured, and so could only see a place in Core Set 2020.
4)
Karn Liberated - Now, Wizard's of the Coast normally doesn't do Planeswalker re-prints in standard legal products, instead opting to create new cards, but I think if there were ever a time to break that rule, War of the Spark would be it. I think Karn, Liberated would probably be the safest to do in this case, because he's only really good because he's the best thing to do with exactly 7 mana in modern.
5)
Phantasmal Image - Another suprisingly expensive card for its power level. Anywhere there is Jace, there can be
phantasmal image, and
Quasiduplicate and Repudiate//Replicate this could make some really janky copying shinanigans.