This is the list of cards that could easily fit into the deck but for one reason or another I have chosen to not play a copy. This also helps me think through other card ideas. Let me know if you have any that you think would be a good fit.
1-2x Reborn Hope: Kinda a multi-colored Snapcaster Mage, but sorcery and need card in graveyard
1x Tolsimir, Friend to Wolves: Versatile threat that works with Huntmaster of the Fells
, but deck already has a higher CMC and found usually have to cut high CMC against aggro decks.
4x Birds of Paradise: speeds up mana production at any color at the cost of reducing the power of Niv ETB trigger, requiring on turn 1, and being a terrible top deck.
4x Gilded Goose: similar to Birds except at least can gain life but less mana since Oko is banned.
2x Thief of Sanity: decent flying threat that generates card advantage, but is a weak body with no immediate impact.
1x Hydroid Krasis: one of the more reasonable cards, haven't tried it out yet. It would replace an Ice-Fang Coatl.
1x Teferi, Hero of Dominaria: Another reasonable card as there are many 2-drop instants but there are already plenty of Azorious cards so had to choose and I chose a lower CMC curve.
3x Spell Queller: Limited protection in the deck so make's it worse on that side of thing than Dovin's Veto, but reasonable.
1x Anguished Unmaking: Might replace Maelstrom Pulse with this card as it is an instant.
4x Manamorphose: Fixes mana and replaces self, but this isn't a storm deck.
1x Abrupt Decay: Very easy to include, Vraska, Golgari Queen would be the replacement.
1x Grim Flayer: Could replace Voice of Resurgence but I liked the multiple creatures over the card advantage.
1x Deputy of Detention: Another possible replacement for Maelstrom Pulse, diversity is the main reason I went with Pulse plus the card stays gone with Pulse.
1x Detention Sphere: Same as Deputy though can't be Bring to Light.
1x Faeburrow Elder: Okay, but CMC too high for a ramp spell.
1x Narset Transcendent: Can't protect self, but a reasonable choice to try out.
1x Nahiri, the Harbinger: Could try this over a Teferi, Time Raveler.
1x Drown in the Loch: Found it doesn't play well with Kaya's Guile and this deck doesn't mill/discard the opponent abundantly enough to be a consistent card.
1x Kambal, Consul of Allocation: I did have one in here, but I replaced it with a Detention Sphere to help against big mana decks. I might put it back in.
1x Cindervines: Same as Kambal.
1x Despark: Limited targets that Trophy or Pulse can't take care of with less downsides.
1x Sigarda, Host of Herons: Currently not playing because Liliana of the Veil decks usually lose to this deck any way. Plus I could only play so many high CMC cards.
1-2x Glittering Wish: I tried this card but that fact that it's a sorcery really hurts this card. Plus after game one I usually would swap it with the actual card in the sideboard so it felt like a waste.
1x Shadow of Doubt: Only a 1-time Ashiok, Dream Render passive.
2-3x Growth Spiral: Already a lot of Simic cards, could be played over Ice-Fang but I feel it is worse as it can't be an attacker/kill spell.
1-2x Knight of the Reliquary: No immediate impact, but would help grab the lands you need especially since the mana base is -focused.
1-2x Qasali Pridemage: The exalted trigger is nice, but the deck is usually not the aggressor.
1-2x Dreadhorde Butcher: Get's blocked too easily for big impact, if was running a higher density of creatures then possible. The deck could be focused on good creatures and then it would possibly fit.
2-3x Once Upon a Time: Reduces Niv's ETB trigger power level and there's a low density of creatures, would instead play a Safewright Quest.
1-2x Saheeli, Sublime Artificer: Too few 1-drops to be really impactful.
1x Kess, Dissident Mage: I was player her for a time but found Niv to just be better as he has a better body and has immediate impact.
1-2x Lavinia, Azorious Renegade: Very possible card if Tron is really common in your meta, it is often found in sideboards of similar decks.
1-2x Guttural Response: 3-drop Teferi already takes care of counter magic and is more than just one turn.
1-2x Safewright Quest: I am thinking about playing it over a land; it is another turn-1 spell and can be grabbed by Niv. Now that the foil is going down in price I am more likely to try this out in the future.
1x Fracturing Gust: Urza and Affinity decks took a hit with Opal being banned. My LGS doesn't have many of these decks so I took it out of my sideboard to add another big mana hoser. Probably still a decent card to have in the side.
2-3x Discovery / Dispersal: Haven't tried it out, just with Discovery was an instant.