I drafted Blackmsmithy's cube a bunch. Here are some thoughs I shared when he asked for advice for UR Spells. I'll divide it in "Core", cards that lead me into the archetype, and "Support", cards that will rank higher if I'm in it but that aren't part of the archetype per se.
Blue: Pteramander, Wavebreak Hippocamp, Niblis of Frost, Ethereal Forager,
Blue Support: Augur of Bolas, Snapcaster Mage
Here the problem is that the most obvious payoffs are very expensive or weak. I think Izzet spells leans towards being aggressive because the better payoffs are and because the longer the game goes, the more spells and card draw you need to keep the deck working.
For example, if I play Niblis of Frost I I need to play at least two spells for it to become decent value. But if I play it on turn 4, chances are I've played other spells before. So I actually need a very high amount of spells and control elements just to make it work. I would try to keep that in mind and add a simpler more aggressive card over Niblis, like Jessian Thief, which is a good blocker for controller decks too.
Also, in my experience, Augur of Bolas is a trap without a very high amount of spells. Same with Glint-Nest Crane for artifacts.
Red Core: Monastery Swiftspear, Burning Prophet, Heartfire Immolator, Young Pyromancer, Guttersnipe
Red Support: Grim Lavamancer
Here I would cut Burning Prophet for sure. It just doesn't hit hard enough. I run just three payoffs in my 52 so you can probably cut Guttersnipe too. After all, the deck is going to have payoffs in other colours as well. Either way, I think Guttersnipe is very fragile and, unlike Young Pyromancer, provides no value so I prefer Mizzium Tank for decks that just want to hit the opponent's face.
White: Soulfire Grandmaster
I don't like the Grandmater. I think it's going to be a 2 mana 2/2 lifelink the vast majority of the time. It leans red much more than it leans blue as a tricolor card and its ability is only relevant for Time Walk and other very undercosted spells. I would rather run Seeker of the Way on its place.
Now, there's an intersection between Izzet Spells and Izzet Draw two and I'm not sure how to judge it. I have similar problems with my cube with discard matters and spells.
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As a whole, I like your cube. It's very elegant and I think the archetypes you have are fun. It's not very different from mine, though I run a far higher power level and yours is "cleaner". I think the main issue with the cube at a glance is that you run too many payoffs and not enough support. You often run 4-5 payoffs for an archetype, which means a good chunk of your cards are not useful to the drafter even if there's good crossover between themes.
