Supreme Verdict: no brainer, a requirement against any kind of aggro.
Hero's Downfall: versatility is very powerful, the ability to target problem creatures or planeswalkers with one card is awesome.
Detention Sphere: used to get rid of assorted permanents,
especially effective against tokens.
Cyclonic Rift: additional instant speed control, overall versatility.
Thoughtseize: lack of counterspells means sculpting the opponent's hand is very appealing.
Sphinx's Revelation: uncertain as to whether 4 are required, but it is the main draw engine in the deck.
Ashiok, Nightmare Weaver: being able to steal an opponents win condition is always nice. With all of the scry, it's fun to play Ashiok after they leave their card on top.
Elspeth, Sun's Champion: mass weenies, board wipe against monsters.
Jace Architect of Thought: helpful for early draw and effect against weenies.
Vraska, the Unseen: vindicate to win.
Aetherling: alternate win condition.
Blood Baron of Vizkopa: mono black and Esper are popular decks, quick to sideboard if against almost anything else.
Obzedat, Ghost Council: annoying and hard to kill creatures are nice.
Elixir of Immortality: effective life gain, reusing cards against another control deck, and helpful against the rogue mill deck.
Initially I included red, but it didn't seem worth it. Rakdos's Return would be very helpful, but without the Nykthos mana engine, the opponent is liable to be low on cards by the time it is worth using. Ral Zarek is also helpful, but it is probably worse than Lightning Strike in this case, since it isn't exactly a win condition and its +1 isn't particularly amazing.
Suggestions are very welcome, anything can be helpful.