Go 5 color with Child of Alara as the commander. Remember it has to go to the graveyard to trigger, so use reanimation effects or recursion like Memorial to Folly , Mortuary Mire , Volrath's Stronghold or Haunted Fengraf to reuse Child. Try to use indestructible cards or utility lands to get all of the effects you want, probably with cards that synergize with lands.
Overall opening this pack was pretty cool. It was suspenseful too since I didn't know what most of the cards were (I did recognize a few by art).
Another thing is to try to make space for Buried Ruin (budget for Academy Ruins) and Haunted Fengraf (budget for Volrath's Stronghold). They can give you additional recursion with Muldrotha as well as crucible of worlds.
Is it just me or is Oriss, Samite Guardian really channeling tatooine bikini Leia. This might be a long one because there's a few things I wanna say and I have a habit of going into way too much detail.
You have very few ways of digging to a second Oriss. Most of your deck doesn't do anything if you don't have at least 2 of her and the chances of that happening naturally in any given game is around 12% on turn three and only reaches 50/50 by turn 12. The planeswalkers, removal and hand disruption help of course however being able to dig for a second copy would be incredibly valuable. I'm thinking Corpse Churn, Discovery / Dispersal or even Shriekhorn seeing as you have so many ways of getting stuff back.
Activating Oriss won't stop them killing her in response. The vast majority of removal spells in the modern format, especially the most played ones, are all instant speed. Modern decks usually fall into 2 categories : Proactive kill you on turn 4 combo decks and "fair" or "linear" control decks that are usually packed with Fatal Pushs(+fetch lands), multiple Lightning Bolts, Terminates, Path to Exiles and Cryptic Commands so if your planning on letting your opponents draw their entire deck they are going to get out of the lock eventually. My suggestion is Lightning Greaves, Swiftfoot Boots or even Gods Willing.
Graveyard hate is a sideboard staple so maybe have some Disenchants ready in the sideboard for any pesky Rest in Peace.
The bit where I suggest random cards. Palace Siege seems insane and tbh you probably want more Liliana, the Necromancer. Codex Shredder is like Shriekhorn except it can be a Time Warp when the lock is partially assembled. Memorial to Folly is a potential alternative to Haunted Fengraf. Dusk / Dawn has potential as both sides can be relevant and you usually want a split on wrath to play around Meddling Mage a.k.a. 1 Damnation, 1 Day of Judgment and 1 Wrath of God instead of 3 Wrath of God.
Anyways sweet deck concept. I think some version of this could be modern viable.
I'd recommend to increase the land count to 36. Those three could be utility lands like Khalni Garden (comes with sac fodder), Haunted Fengraf (built-in recursion), Mortuary Mire (same), or Bojuka Bog (hose opponents' graveyard strategies, especially if you help them by accident with sth like Ghoulcaller's Bell).
With your Undergrowth theme, I'd switch some of the sorceries that mill yourself with creatures that do so, e.g. Balustrade Spy, Returned Centaur, Sibsig Host. Also, where are Bitter Revelation and Dark Bargain?
If you really like Treshold, Krosan Restorer and Toxic Stench would have been my first two picks. Cut those that just grow in power/toughness for them.
For the sake of saving words I'm just going to list my cuts and my adds. Then feel free to ask any questions.