Great deck! I've been fascinated with optimizing a Niv-Mizzet Reborn deck ever since the card was spoiled. I was disappointed by the number of non-two-color cards in the initial deck lists I saw.
The decks have gone in slightly different directions, but they both fall in to the category of 2-color good stuff. I'll highlight some of the differences, but I don't mean to imply that my list is better. I'm just highlighting my thoughts.
Non Guild cards: I left out sol ring and the like in favour for all lands. My logic was, Mana Crypt and Sol Ring don't help cast Niv-Mizzet Reborn so more lands would be better. I have 39 lands and exactly 6 cards of each guild. I have Mistveil Plains in the deck, My logic being to add the 2-color spells back to the library and use the shuffle effects to improve Niv-Mizzet Reborn triggers, However it might be too subtle to be worth it.
Borderposts: I'm running 5 basics and it still felt like I always needed to pay 3 for these, also I did not like the bounce because does it not actually accelerate. I found enough other mana cards to avoid them.
Azorius: I have less flicker/blink effects and more targets. Elite Guardmage is a nice target for the flicker effects and Unbender Tine is the Azorious version Kiora's Follower. I lack reanimation in my build so I avoided Vanish into Memory. I have less Planeswalkers so Oath of Teferi is also missing.
Dimir: Diabolic Vision, Psychic Symbiont, and Recoil are cards in my list, but Countersquall/Undermine may be better cards. Diabolic Vision can be a used to set up a Niv-trigger.
Rakdos: I went with ETB creatures to fill out this color pair: Hellhole Rats, Murderous Redcap, Rakdos Firewheeler, and Sanity Gnawers.
Gruul: I'm surprised not to see Wild Cantor and Draconic Disciple. I went with Raging Regisaur and Regisaur Alpha for a small dinosaur theme. (Atzocan Seer helps out.) Rhythm of the Wild is nice here, with blink effect creatures can get both haste and +1/+1 counter. (haste, attack, blink, +1/+1). I wish I had room for both Radha, Heir to Keld,and Zhur-Taa Druid, but I used one preferring the higher mana cost of Draconic Disciple to get the "any color of mana".
Selesnya: Atzocan Seer, Eladamri's Call, Knight of New Alara, Reborn Hope, and Steward of Valeron. Privileged Position is the only card in both lists. I avoided Mirari's Wake since it does not help cast Niv-Mizzet. I like Safewright Quest, but I felt I had enough lands that did this effect. I'd like to find room for Aura Shards. Knight of New Alara makes Niv-Mizzet Reborn an 11/11 flyer and all the other non-token creatures get +2/+2
Orzhov: Angel of Despair is a great target for Mistmeadow Witch or Venser. I went with Vindicate over Anguished Unmaking just to be able to hit problematic lands. I still need to get Kaya, Ghost Assassin, but I plan to add her when I do.
Izzet: Cloven Casting I felt belonged in the deck. Hypersonic Dragon lets me feel better about sorcery spells I put in. I went with more damage spells here over the counter magic: Electrolyze and Prophetic Bolt. Fire/Ice would be nice too, but I did not make room for it, I wanted Counterflux in.
Golgari: Very similar. I have Golgari Findbroker, Assassin's Trophy, and Pernicious Deed, instead of Charm, Journey, and Bloom.
Boros: Sky Terror is an nice quick creature with 2 evasion abilities. Good for taking out Planeswalkers, or at least keeping them from ultimate. Aurelia, the Warleader and Deflecting Palm are cards excluded from your list.
Simic: I love Bring to Light, Niv-Mizzet Reborn sees a two color spell, but I see a 5 color spell that does exactly what I need it to. Kruphix, God of Horizons, I added to avoid discarding, which is possible sub-optimal. Temporal Spring is another "target permanent" sorcery I went with for the versatility.
I plan to do some more experimenting. Thanks again for sharing you lists! I hope you find my comments useful or at least interesting.
I have an alternate "Master of Guilds" build here: Maximized Niv-Mizzet Reborn
The decks have gone in slightly different directions, but they both fall in to the category of 2-color good stuff. I'll highlight some of the differences, but I don't mean to imply that my list is better. I'm just highlighting my thoughts.
Non Guild cards: I left out sol ring and the like in favour for all lands. My logic was, Mana Crypt and Sol Ring don't help cast Niv-Mizzet Reborn so more lands would be better. I have 39 lands and exactly 6 cards of each guild. I have Mistveil Plains in the deck, My logic being to add the 2-color spells back to the library and use the shuffle effects to improve Niv-Mizzet Reborn triggers, However it might be too subtle to be worth it.
Borderposts: I'm running 5 basics and it still felt like I always needed to pay 3 for these, also I did not like the bounce because does it not actually accelerate. I found enough other mana cards to avoid them.
Azorius: I have less flicker/blink effects and more targets. Elite Guardmage is a nice target for the flicker effects and Unbender Tine is the Azorious version Kiora's Follower. I lack reanimation in my build so I avoided Vanish into Memory. I have less Planeswalkers so Oath of Teferi is also missing.
Dimir: Diabolic Vision, Psychic Symbiont, and Recoil are cards in my list, but Countersquall/Undermine may be better cards. Diabolic Vision can be a used to set up a Niv-trigger.
Rakdos: I went with ETB creatures to fill out this color pair: Hellhole Rats, Murderous Redcap, Rakdos Firewheeler, and Sanity Gnawers.
Gruul: I'm surprised not to see Wild Cantor and Draconic Disciple. I went with Raging Regisaur and Regisaur Alpha for a small dinosaur theme. (Atzocan Seer helps out.) Rhythm of the Wild is nice here, with blink effect creatures can get both haste and +1/+1 counter. (haste, attack, blink, +1/+1). I wish I had room for both Radha, Heir to Keld,and Zhur-Taa Druid, but I used one preferring the higher mana cost of Draconic Disciple to get the "any color of mana".
Selesnya: Atzocan Seer, Eladamri's Call, Knight of New Alara, Reborn Hope, and Steward of Valeron. Privileged Position is the only card in both lists. I avoided Mirari's Wake since it does not help cast Niv-Mizzet. I like Safewright Quest, but I felt I had enough lands that did this effect. I'd like to find room for Aura Shards. Knight of New Alara makes Niv-Mizzet Reborn an 11/11 flyer and all the other non-token creatures get +2/+2
Orzhov: Angel of Despair is a great target for Mistmeadow Witch or Venser. I went with Vindicate over Anguished Unmaking just to be able to hit problematic lands. I still need to get Kaya, Ghost Assassin, but I plan to add her when I do.
Izzet: Cloven Casting I felt belonged in the deck. Hypersonic Dragon lets me feel better about sorcery spells I put in. I went with more damage spells here over the counter magic: Electrolyze and Prophetic Bolt. Fire/Ice would be nice too, but I did not make room for it, I wanted Counterflux in.
Golgari: Very similar. I have Golgari Findbroker, Assassin's Trophy, and Pernicious Deed, instead of Charm, Journey, and Bloom.
Boros: Sky Terror is an nice quick creature with 2 evasion abilities. Good for taking out Planeswalkers, or at least keeping them from ultimate. Aurelia, the Warleader and Deflecting Palm are cards excluded from your list.
Simic: I love Bring to Light, Niv-Mizzet Reborn sees a two color spell, but I see a 5 color spell that does exactly what I need it to. Kruphix, God of Horizons, I added to avoid discarding, which is possible sub-optimal. Temporal Spring is another "target permanent" sorcery I went with for the versatility.
I plan to do some more experimenting. Thanks again for sharing you lists! I hope you find my comments useful or at least interesting.