Welcome to the cEDH forums, Rowan. I always knew your Niv-Mizzet deck would find its way here, haha.
First, I want to encourage you to read the Philosophy doc at the top of the forums, if you haven't already done that. I think it's very important for you to accept this mindset since, based on the comments in the OP, previous comments you've made in your Niv thread, and comments other threads, you ARE concerned about the emotional well-being of the table. While I applaud this and whole-heartedly support this mentality on a general basis, cEDH is a totally different philosophy. It's all about winning with the best cards available and playing as optimally as possible. This can come off as being ruthless or insensitive, which can create emotional feel-bads for the rest of the table, but it's simply playing your deck as efficiently and effectively as possible at all times. Posting here is an invitation and implied recognition that you're looking for optimal suggestions based solely on power level and mana efficiency.
Now that this is out of the way, there are two principles that are important to understand about cEDH: Curve Considerations & Mana Advantage. cEDH decks are inherently trying to break/abuse/circumvent the normal rules of Card and Mana Advantage, similar to what Legacy or Vintage decklists are doing. Consequently, every cEDH decklist is looking for the best type of effect at the lowest CMC possible so that you can play the highest number of spells per turn cycle possible. This generally means that there is a higher density of non-creature spells in the decks than normal. With this understanding, the card suggestions I make will have some context so that you can evaluate them properly. Seeing that you're coming from a period of inactivity, there are some awesome and exciting cards that you should certainly include.
Mystic Remora is quite simply one of the best cEDH draw spells in the entire format. No one can ever pay 4 to prevent you from drawing, so it regularly draws upwards of 4+ cards EVERY TIME YOU CAST IT. And if you don't draw that number of cards, then your opponents probably aren't casting spells, which is a good thing for you.
Mox Diamond and Mana Crypt are notable exclusions that I'm assuming exist because of budget reasons. If your playgroup is cool with proxies, you should certainly proxy them up. Fast Mana rocks are the name of the game in cEDH because it allows you to generate Mana Advantage faster than traditional ramp methods and land drops.
Pyroblast and its cousin Red Elemental Blast are super strong inclusions. Blue is the best color in cEDH (rightly so!), so there will almost ALWAYS be a Blue player at the table, more often 2+. A one mana Counterspell/Vindicate is quite powerful.
Curiosity is one of the best win conditions you can have, playing Niv. It costs 1 CMC and will immediately trigger when someone casts an instant/sorcery. In my mind, including a few of the Curiosity effects is a must for Niv because it's such a compact way to win the game.
Force of Will because duh. Proxy it if you can, otherwise I recommend putting it on your acquire list. Force of Negation is a great complement/substitute to FoW, but it's no FoW.
I recommend cutting pretty much all of the high CMC cards in your list. You're frequently not going to have enough time to cast Omniscience and, particularly in the case of Omniscience, there are better cards at lower CMCs that do just as good if not better things than what you're currently running.
Inner Fire is cute and inefficient, as is Alhammarret's Archive.
A good starting list can be found here. While there are some meta choices that may not be effective for you (i.e. Pyroclasm effects), this is a streamlined list that showcases the principles and mindset cEDH requires.
A list that I would personally start with would look something more like this:
First, I want to encourage you to read the Philosophy doc at the top of the forums, if you haven't already done that. I think it's very important for you to accept this mindset since, based on the comments in the OP, previous comments you've made in your Niv thread, and comments other threads, you ARE concerned about the emotional well-being of the table. While I applaud this and whole-heartedly support this mentality on a general basis, cEDH is a totally different philosophy. It's all about winning with the best cards available and playing as optimally as possible. This can come off as being ruthless or insensitive, which can create emotional feel-bads for the rest of the table, but it's simply playing your deck as efficiently and effectively as possible at all times. Posting here is an invitation and implied recognition that you're looking for optimal suggestions based solely on power level and mana efficiency.
Now that this is out of the way, there are two principles that are important to understand about cEDH: Curve Considerations & Mana Advantage. cEDH decks are inherently trying to break/abuse/circumvent the normal rules of Card and Mana Advantage, similar to what Legacy or Vintage decklists are doing. Consequently, every cEDH decklist is looking for the best type of effect at the lowest CMC possible so that you can play the highest number of spells per turn cycle possible. This generally means that there is a higher density of non-creature spells in the decks than normal. With this understanding, the card suggestions I make will have some context so that you can evaluate them properly. Seeing that you're coming from a period of inactivity, there are some awesome and exciting cards that you should certainly include.
Mystic Remora is quite simply one of the best cEDH draw spells in the entire format. No one can ever pay 4 to prevent you from drawing, so it regularly draws upwards of 4+ cards EVERY TIME YOU CAST IT. And if you don't draw that number of cards, then your opponents probably aren't casting spells, which is a good thing for you.
Mox Diamond and Mana Crypt are notable exclusions that I'm assuming exist because of budget reasons. If your playgroup is cool with proxies, you should certainly proxy them up. Fast Mana rocks are the name of the game in cEDH because it allows you to generate Mana Advantage faster than traditional ramp methods and land drops.
Pyroblast and its cousin Red Elemental Blast are super strong inclusions. Blue is the best color in cEDH (rightly so!), so there will almost ALWAYS be a Blue player at the table, more often 2+. A one mana Counterspell/Vindicate is quite powerful.
Curiosity is one of the best win conditions you can have, playing Niv. It costs 1 CMC and will immediately trigger when someone casts an instant/sorcery. In my mind, including a few of the Curiosity effects is a must for Niv because it's such a compact way to win the game.
Force of Will because duh. Proxy it if you can, otherwise I recommend putting it on your acquire list. Force of Negation is a great complement/substitute to FoW, but it's no FoW.
I recommend cutting pretty much all of the high CMC cards in your list. You're frequently not going to have enough time to cast Omniscience and, particularly in the case of Omniscience, there are better cards at lower CMCs that do just as good if not better things than what you're currently running.
Inner Fire is cute and inefficient, as is Alhammarret's Archive.
A good starting list can be found here. While there are some meta choices that may not be effective for you (i.e. Pyroclasm effects), this is a streamlined list that showcases the principles and mindset cEDH requires.
A list that I would personally start with would look something more like this:
6x Niv-Mizzet, Parun
Mana Sources (15)
0x Mana Crypt
0x Chrome Mox
0x Mox Diamond
0x Mox Opal
1x Sol Ring
1x Mana Vault
2x Izzet Signet
2x Talisman of Creativity
2x Fellwar Stone
2x Coldsteel Heart
2x Desperate Ritual
2x Grim Monolith
4x Phyrexian Metamorph
4x Treasonous Ogre
5x Gilded Lotus
Combo Pieces (7)
1x Curiosity
2x Isochron Scepter
2x Dramatic Reversal
3x Ophidian Eye
3x Tandem Lookout
5x Paradox Engine
6x Thousand-Year Storm
Win Conditions (2)
4x Aetherflux Reservoir
6x Expansion // Explosion
Tutors (8)
1x Mystical Tutor
1x Dizzy Spell
2x Transmute Artifact
2x Reshape
3x Fabricate
3x Intuition
3x Spellseeker
3x Whir of Invention
0x Pact of Negation
1x Flusterstorm
1x Pyroblast
1x Red Elemental Blast
1x Swan Song
2x Counterspell
2x Mana Drain
2x Muddle the Mixture
2x Narset's Reversal
3x Disallow
3x Force of Negation
5x Force of Will
Card Draw (14)
1x Mystic Remora
1x Brainstorm
1x Preordain
1x Ponder
1x Sensei's Divining Top
2x Pull from Tomorrow
3x Rhystic Study
3x Narset, Parter of Veils
3x Windfall
3x Wheel of Fortune
3x Whirlpool Warrior
5x Future Sight
6x Echo of Eons
6x Time Spiral
Removal/Stax (10)
1x Grafdigger's Cage
1x Lightning Bolt
1x Rapid Hybridization
1x Chain of Vapor
1x Rolling Earthquake
2x Cyclonic Rift
2x Reality Shift
2x Gilded Drake
2x Cursed Totem
3x Chaos Warp
1x Inventors' Fair
1x Ancient Tomb
1x Command Tower
1x City of Brass
1x Reflecting Pool
1x Forbidden Orchard
1x Volcanic Island
1x Steam Vents
1x Fiery Islet
1x Shivan Reef
1x Sulfur Falls
1x Cascade Bluffs
1x Spirebluff Canal
1x Cephalid Coliseum
1x Scalding Tarn
1x Polluted Delta
1x Flooded Strand
1x Misty Rainforest
1x Wooded Foothills
1x Prismatic Vista
1x Bloodstained Mire
1x Arid Mesa
6x Island
2x Mountain
Hopefully this helps!
UB Dralnu, Lich Lord
RBW [Primer]-Kaalia of the Vast
BUG [Primer]-Tasigur, the Golden Fang
GWU [Primer]-Arcades, the Strategist
WUB Primer-Aminatou, the Fateshifter
UBR Nicol Bolas, the Ravager