Spellthief (Rogue Talent)
Special: Unlike most class talents, this talent takes up two rogue class talent slots. You must be single-classed.
You gain a free 5 point arcane background to represent your Spellthief training. You also gain two Wizard spells, which you cast as if you were a Wizard of the same class level. You can change these spells during a full heal-up. Finally, regardless of your level, you gain the bonus power Spellthief's Strike (below) in addition to your normal number of powers.
Talk to your DM if your would like your character to learn his spells from a different class, such as Sorcerer, if you feel that it would fit the PC's background or One Unique Thing better.
Adventurer feat: You gain three cantrips of your choice from the wizard list. You can cast them like a wizard (but without the benefits of Cantrip Mastery).
Champion feat: You gain an additional Wizard spell, at 2 levels lower than your class level.
Epic feat: You gain a second Wizard spell at 2 levels below your class level.
Spellthief Strike (Rogue power)
Note: This is a bonus power for rogues with the Spellthief talent. Rogues who are multiclassed with a spellcasting class can choose it as a standard 1st-level power if they like.
Special: You must have momentum and be able to deal your Sneak Attack damage to the target if you hit.
Target: One enemy
Attack: Dexterity + Level vs. PD
Hit: Half of WEAPON + Dexterity damage (including Sneak Attack damage). The target is hampered until the end of its next turn. You can expend your momentum to regain the use of one spell of your choice. As a big exception to usual multiclass rules, you can regain spells of your other class when multiclassed. If the target is a spellcaster, you can steal a spell from the target instead of regaining your own spell.
Natural 18+: You do not need to expend momentum to regain or steal a spell.
Adventurer feat: You can choose to recharge a magic item power instead of regaining a spell.
Champion feat: On 18+, the target is hampered (save ends).
Epic feat: The attack deals full damage on a hit.
This is an attempt to reimagine the Spellthief class from the 3.5 Complete Adventurer in 13th Age. The draft is (loosely) based on the Rogue's Thievery talent and Thief's Strike.
The reason Spellthief Strike requires momentum is to require some tactics and setup. You can't just spam Fireball - Spellthief Strike - Fireball - Spellthief Strike, at the very least least you need a hit to regain momentum in between.
The reason why multiclassed Rogue/Wizards can't get the talent is that they already have near-full Wizard spells. Giving them two more slots on top is overkill.