It depends on what you want that slot to do. Another sweeper, maybe? Or a counterspell. For making casual players less salty, I've recently become a fan of Dream Fracture. The downside is minimal in this format.

What's another cheap mass board wipe? All Is Dust maybe? Or Oblivion Stone? They'd both get rid of artifacts and enchantments which is something Dimir colors have trouble doing,

Nevinyrral's Disk works, too.

Indeed. Doesn't hit planeswalkers though. But it is cheaper. Lol. Choices choices..

If you are looking for boardwipes, I personally love Decree of Pain, Life's Finale, and Deadly Tempest.

Also, I'd suggest a couple more sac outlets to control the board through Grave Pact, Dictate of Erebos, and Living Death. Ashnod's Altar and Viscera Seer are great because they have no activation cost.

Altar of Dementia is good too. You could target yourself.

I was thinking of using Sadistic Hypnotist or Phyrexian Plaguelord for the sac outlets. Since they're both creature and can be tutored into the graveyard and recurred easily. Viscera Seer could be good too, scrying my whole deck to find what I need.

Speaking of scrying your whole deck, Lim-Dul's Vault does exactly that hehe

I have that on paper, but not on here apparently. The decks aren't exactly the same yet. But it is a great card.

I just put together my deck: Gisa and Geralf: Siblings of Self-Mill. Please take a look and let me know what you think :)