I've been thinking about this concept deck for a while now, but have not figured out a good shell to start testing at an FNM with. So if you are down to brew, I'd love to hear your thoughts as I haven't come across this deck idea yet on Reddit, the forums or Youtube yet although I'm sure others are looking at similar interactions. So first off Dreadhorde Arcanist is a powerful card when you have a bunch of spells that it can recur and also is simply a fine blocker against fast aggro. Grixis has Opt and Shock as 1 drops relevant and good with Arcanist, but also in the new set is Scheming Symmetry. For us Shock is to stay alive, Opt is to set up, and Scheming as a combo piece. The combo I am most interested in is Bolas's Citadel. If Dreadhorde Arcanist and Bolas's Citadel are in play, you cast Scheming Symmetry tutoring Thousand-Year Storm to the top of your deck, then pay 6 life to cast it. Then you move to attacks and attack with Dreadhorde Arcanist targeting the Scheming Symmetry you cast earlier in your graveyard which will create an additional copy withThousand-Year Storm. The first Scheming Symmetry will resolve, with the second on the stack allowing you are opportunity to cast an instant you fetched from the top of your deck, I am going to assume here that the opponent has a bit of a board presence and tutor Moment of Craving, casting it for 2 life (8 total for the turn with the Thousand-Year Storm losing 6 earlier), and making two additional copies from Thousand-Year Storm. This will gain you 6 life back (-2 for turn). At this point the second copy of Scheming Symmetry will resolve, and you will search up another copy of Scheming Symmetry which you cast for 1 life (-3 for turn), and Thousand-Year Storm will create 3 additional copies. With three copies on the stack, one resolves finding Blink of an Eye (make 4 additional copies, -4 life for turn), as an example, to return hate cards like Leyline of Sanctuary to the opponents hand (although obviously this hate card needed to be bounced earlier given Scheming Symmetry needs to target them, but maybe Narset, Parter of Veils or something needs to be returned -- I think bounce is probably still important to include as a mention), then the next Scheming Symmetry resolves with 2 more on the stack. Shock at this point makes 5 additional copies, so Shocking them out should be on the table with Shock as the next Symmetry target as well. If shock isn't an option maybe a white splash is in the deck with Depose // Deploy with 5 copies making 2 thopters, then gaining 3 life, then 2 thopters, then gaining 5 life, then 2 thopters, then gaining 7 life, then 2 thopters, then gaining 9 life, then 2 thopters, then gaining 11 life, then 2 thopters, then gaining 13 life, repeat with a second copy to have 26 thopters at the ready next turn (there is take extra turn cards like Chance for Glory and Nexus of Fate which then makes this likely lethal on your extra turn's attack phase). Or burn works, but just used the shocks early game Ral's Outburst with 5 copies would be 18 damage and a lot of digging which should get you the last points to cast from hand (you spent 1 mana this turn) or the additional Symmetry's will find another burn spell to close it. Simply Nexus for 6 turns, or for 7 turns, or Nexus for 8 turns then likely makes attacking with your 1/3 Dreadhorde lethal as well if you bounced their board with Blink of an Eye and have Shocks in your graveyard (Nexus, then Moment of Craving, then Nexus again would work I assume given enough targets for moment for your opponent). I get that this is a 2 drop surviving, a 6 drop artifact in play, and then a Symmetry in hand to win, but it seems like many cards that fit into the combo finish would simply be good in a Grixis Control list. I feel like a starting shell would look like the following:
That shell of core cards to start is 26 cards, which allows you wiggle room to customize the Grixis Control deck to more controlling or potentially maybe adding a reanimator shell on top of it. For example with a reanimator look you could fill it out with Tomebound Lich, Blood for Bones, Search for Azcanta, Radical Idea, Nicol Bolas, the Ravager, and a reanimation target like Vilis, Broker of Blood or Niv-Mizzet, Parun. I feel though, that Grixis control is one of the better starting places as even a value Bolas's Citadel that gets "stuck" on something like a counter on top is a decent surprise for the opponent.
Which would be 35 cards, with 25 (26 at 61 card deck) lands which should be close to good with the Opts. Enter the God-Eternals would be a nice finisher to the combo as well with the final Scheming Symmetry and worth looking at in the sideboard at the least. I think additional Ritual of Soot in the sideboard as well as at least a copy of Legion's End in the board for aggro and Counterspells and potentially more discard for control would be smart, Duress (one drop can be cast off the Dreadhorde), Thought Erasure, Negate, Disdainful Stroke (though I would be scared of these against the little Teferi). Some answers to Thrashing Brontodon such as Cast Down would also be high on my considerations for the board.
Like I opened with, I'm not sure how good this concept would be and if you have any ideas on improvement or even a different direction to take it in, I'm listening. (One more thing to note about Enter the God-Eternals, I thought it would be hilarious as the finisher due to milling their tutored card with the final Symmetry, but the idea of them not having a creature to target scares me off as a maindeck option for the first path. I feel like using Moment of Craving to build up life, then Blink to bounce their board, then Nexus a ton for enough turns to lethal them with dreadhorde is one of the safest lines. You can replace Blink with the Dispersal half to kill their hand as well if you have to pass turn (lower on life than Nexus). The new Mythic Brainstorm Horse I think could replace Kefnet pretty easily as the beefy creature of the deck that can also block a medium to large flying threat.
I would run 4 copies of Bolas's Citidel as it's one of your combo pieces. With 4 of each of the 3 combo pieces, you roughly have a 25% chance of this being able to go off on turn 6 (that's with no mulligans as mulliganing to your 2 or 3 of your combo pieces is highly unlikely.) Therefore, you need either more tutors or loads of card draw. (I would suggest more tutors.)
Options for tutors: Mastermind's Acquisition is a great tutor and will get you any card you need. It can even get cards from your sideboard once the combo's gone off if you need a particular card to hose a particular deck. Consider running 3 or 4 of it. Mausoleum Secrets might work if you go reanimator, but you could only really use it to tutor for Dreadhorde Arcanist or Scheming Symmetry.
Also, I would consider running Finale of Promise as well as Dreadhorde Arcanist as it can do the same job. This might take up too many cards though and make it harder to survive the early game.
4 Dreadhorde Arcanist
Sorcery (4)
4 Scheming Symmetry
Instant (13)
4 Opt
4 Shock
2 Moment of Craving
2 Blink of an Eye
2 Ral's Outburst
1 Nexus of Fate
2 Bolas's Citadel
Enchantment (1)
1 Thousand-Year Storm
That shell of core cards to start is 26 cards, which allows you wiggle room to customize the Grixis Control deck to more controlling or potentially maybe adding a reanimator shell on top of it. For example with a reanimator look you could fill it out with Tomebound Lich, Blood for Bones, Search for Azcanta, Radical Idea, Nicol Bolas, the Ravager, and a reanimation target like Vilis, Broker of Blood or Niv-Mizzet, Parun. I feel though, that Grixis control is one of the better starting places as even a value Bolas's Citadel that gets "stuck" on something like a counter on top is a decent surprise for the opponent.
4 Dreadhorde Arcanist
1 God-Eternal Kefnet
Sorcery (11)
4 Scheming Symmetry
2 Thought Erasure
2 Discovery // Dispersal
2 Cry of the Carnarium
1 Ritual of Soot
4 Opt
4 Shock
2 Moment of Craving
1 Blink of an Eye
1 Bedevil
1 Ral's Outburst
1 Nexus of Fate
3 Narset, Parter of Veils
1 Nicol Bolas, Dragon-God
Artifact (2)
2 Bolas's Citadel
Enchantment (1)
1 Thousand-Year Storm
Which would be 35 cards, with 25 (26 at 61 card deck) lands which should be close to good with the Opts. Enter the God-Eternals would be a nice finisher to the combo as well with the final Scheming Symmetry and worth looking at in the sideboard at the least. I think additional Ritual of Soot in the sideboard as well as at least a copy of Legion's End in the board for aggro and Counterspells and potentially more discard for control would be smart, Duress (one drop can be cast off the Dreadhorde), Thought Erasure, Negate, Disdainful Stroke (though I would be scared of these against the little Teferi). Some answers to Thrashing Brontodon such as Cast Down would also be high on my considerations for the board.
Like I opened with, I'm not sure how good this concept would be and if you have any ideas on improvement or even a different direction to take it in, I'm listening. (One more thing to note about Enter the God-Eternals, I thought it would be hilarious as the finisher due to milling their tutored card with the final Symmetry, but the idea of them not having a creature to target scares me off as a maindeck option for the first path. I feel like using Moment of Craving to build up life, then Blink to bounce their board, then Nexus a ton for enough turns to lethal them with dreadhorde is one of the safest lines. You can replace Blink with the Dispersal half to kill their hand as well if you have to pass turn (lower on life than Nexus). The new Mythic Brainstorm Horse I think could replace Kefnet pretty easily as the beefy creature of the deck that can also block a medium to large flying threat.
Options for tutors:
Mastermind's Acquisition is a great tutor and will get you any card you need. It can even get cards from your sideboard once the combo's gone off if you need a particular card to hose a particular deck. Consider running 3 or 4 of it.
Mausoleum Secrets might work if you go reanimator, but you could only really use it to tutor for Dreadhorde Arcanist or Scheming Symmetry.
Also, I would consider running Finale of Promise as well as Dreadhorde Arcanist as it can do the same job. This might take up too many cards though and make it harder to survive the early game.
With 4 Dreadhorde Arcanist, 4 Finale of Promise, 4 Bolas's Citadel, 4 Scheming Symmetry and 4 Mastermind's Acquisition, this combo should be able to go off on turn 6 roughly 79% of the time (if my maths is correct.) Try to fill the rest of the deck with kill / burn / board wipe spells to keep your opponent under control till turn 6.
I hope these suggestions are useful.