The Unseen Seer was one of my favorite prestige classes of 3.5, and here I try to recreate it as a Magus Archetype. I hope you like it and find it balanced, or give me hints on how to make it better!
The Unseen Seer
Always elusive, an unseen seer can either be a lethal assassin or a trader in secret. She uses magic for subterfuge and to help her steal secrets, remaining hidden or even invisible.
Roguish training
The Unseen Seer gains 2 more skill points per level. She also adds Acrobatics, Disable Device, Perception, Stealth and Sleight of Hand to her list of class skills.
This ability replaces Fighter Training. She also never gains Medium or Heavy Armor Proficiency, and cannot cast spell in medium or heavy armor without incurring in arcane spell failure.
Advanced Learning
Starting at first level, and every three levels after that, the Unseen Seer can add a Divination or Illusion spell (except those of the Shadow subschool) from any list to her list of class spells. The spell added must be of a level the Unseen Seer can already cast.
This ability replaces Greater Spell Access.
Sneak Attack
At 4th level, an unseen seer inflicts +1d6 points of damage against targets within 30 feet that she flanks or that are denied their Dex bonus to AC against her. This damage increases by +1d6 every four levels after 4th.
This ability replaces Spell Recall and Improved Spell Recall.
Silent Spell
At 5th level the Unseen Seer gains Silent Spell as a bonus feat.
This replaces her fifth level bonus feat.
Guarded Mind
At 9th an unseen seer is protected by a nondetection effect (like the spell, but permanent). Her caster level for this effect is equal to her class level. If the effect is dispelled, she can reactivate it as a standard action.
This ability replaces her 9th level Magus Arcana.
Of course I forgot an important part and now can't edit my post anymore. Anyway...
Divination Power
At 5th level the Unseen Seer focuses her power on divination. She gains +1 to her caster level when casting any spell from the Divination school. This power come at a cost: whenever she casts a spell from another school, her caster level is 1 lower.
At 10th level the bonus and penalty grow to 2; at 15th level, they grow to 3. Finally at 20th level they grow to 4.