There’s only 20 blue creatures in the cube, which sounds roughly right. Blue is supposed to be a spell color (in fact, the most spell intensive), so this is good. The curve is little lacking, but blue isn’t the kind of color that really needs one. I’ll try to fix it, though. The color also needs one or two cards in the latter part of the curve.
Otherwise, counters, card draw and maybe a trick or two.
The current curve is 2 / 6 / 4 / 6 / 2, so assuming I’ll add eight creatures, the right way to do it might be 1 / 1 / 3 / 0 / 2 / 1, which would leave me with a curve of 3 / 7 / 7 / 6 / 4 / 1, but I’d rather put [scryfall]Conundrum Sphinx[/scryfall] in than one more five-drop, so the final curve will be 3 / 7 / 7 / 7 / 3 / 1.
For a one-drop, I suppose [scryfall]Delver of Secrets[/scryfall] might be a good addition. It obviously isn’t anywhere as strong as it is in constructed, but still. A fine ability that synergizes well with all the Scrying.
For a two-drop, I’d like to put in [scryfall]Artisan of Forms[/scryfall], because it can be fun, if you can get a nice deck around it. Besides, copying that bomb your opponent has, has a tendency to level the battlefield.
[scryfall]Illusory Angel[/scryfall] doesn’t really fit the curve, because of its drawback, but its in. [scryfall]Meletis Charlatan[/scryfall] is my old nemesis (there always seems to be one rare you open all the time in each set and for Theros, this was it for me), but I still like the ability, so its in. [scryfall]Mizzium Meddler[/scryfall] is a good blocker, with excellent, if situational abilities.
I already mentioned the [scryfall]Conundrum Sphinx[/scryfall], so lets skip four-drops.
For five-drops, [scryfall]Perplexing Chimera[/scryfall], because of the weird effect it has on games. Its also a good, generic answer to pretty much anything, if you can get it out early enough.
[scryfall]Sphinx of Magosi[/scryfall] seems like pretty quintessential blue fatty, so I’ll use the sole six-drop spot for that.
Blue isn’t much into destroying anything (actually, not destroying anything is part of its philosophy), but it can counter and bounce. It can also steal stuff and limit opponents ability to do damage.
[scryfall]Essence Scatter[/scryfall] is a good basic counter, but I’ll also put in its opposite number, [scryfall]Negate[/scryfall]. What else? [scryfall]Arcane Denial[/scryfall]? [scryfall]Dream Fracture[/scryfall]? I’ll go with [scryfall]Mana Leak[/scryfall], [scryfall]Disdainful Stroke[/scryfall] and [scryfall]Stubborn Denial[/scryfall], which leaves us with only a couple of hard counters, but that’s fine.
[scryfall]Supplant Form[/scryfall] is quite expensive, but can also be an effective trick. Its not a real answer, but blue should be able to use the extra one round without that one bomb in the field making things difficult.
[scryfall]Dampening Pulse[/scryfall] is a good card any aggressive decks. There aren’t too many creatures in the cube it completely nullifies, but even slowing down clocks is good. Against the decks that go wide, its very good.
[scryfall]Displacement Wave[/scryfall] can clear the board nicely. I’ll put it in, because its so much more versatile than many of the other mass bounce spells.
[scryfall]Mirror Mockery[/scryfall] is sort of an answer, but it can also be used nicely, if you have ETB-effects, especially on creatures that have evasion.
Tricks and Others
[scryfall]Clockspinning[/scryfall] can sometimes be a trick, but at other times it can be just the spell you need to win the game, if you can set it up nicely
[scryfall]Military Intelligence[/scryfall] is a card I enjoy.