I think most people would agree that sorcery spell gems are one of the least attractive when it comes to pure damage, which is the exact opposite of what people expect from this class type. The utility of sorcery spell gems is also debatable but I won’t go into that too much as I don’t have much experience with what makes utility spells good. Anyway here are some of my thoughts:
Make shatter a full party spell instead of single target. This gives it a bit more synergy with Flash Freeze and also some nature synergy with Snowstorm.
Rework spell breath/spell strike. While the int/attack split is interesting on paper, I feel it’s too niche in practice and doesn’t reward you adequately for splitting your stats. You could perhaps make it so the total stat value from int and speed adds up to 120% or something but again I don’t feel like it’s a very interesting mechanic. I would rather see something like +damage dealt bases on enemy’s number of debuffs or even player’s number of buffs. Or even just increased damage with buffs like Arcane/ward.
Magic missile could use a slight increase somehow. 0 mana cost on the weakest damage spell isn’t all that great thanks to how many ways you can get mana or get 0 cost spell gems. Maybe move it up to the next damage tier and it would be kinda cool if it came with Casts on Extra Target by default, with the potential to hit up to 3 creatures? That would also give it a bit more classic “multi hit” magic missile feel.
Sleep in general is very under utilized and sorcery seems to be the main way to afflict it. Could be another opportunity to add synergy between spells but I can’t think of anything specific at the moment.
Edit: forgot spell breath/strike was attack and not speed, corrected