Something I'm considering for an upcoming game:
Take the Rogue, chained or un, and replace rogue talents with sphere talents, but with a limited selection of possible spheres (telekinesis, illusion... Maybe dark), and a mid caster level progression.
Would fans of arcane trickster try this?
You should look at the basic and advanced magical training feats in the feats section of spheres of power. I'd simply allow those feats plus extra spell pool and extra talent (telekinesis, illusion, dark) all as rogue talents.
The Hedge Witch comes about as close as it gets to a good fusion of spells and trickery with the Academia and Charlatanism tradition combo, and anything you add would have to compete with that. They get more spell points, more talents, more skills thanks to Versatile Performance, and can access everything the chained Rogue has besides Uncanny Dodge. By comparison an UnRogue with limited talent options and less spell points isn't so much of an "Arcane" Trickster as it is a Rogue with some neat tricks.
The prestige class attempts to mix spells and subterfuge by granting you some magical abilities that let you be flexible. They can disable traps and pick pockets from across a room, sneak attack a target without meeting the requirements, cast a few spells per day silent and stilled, and can even sneak attack with area spells. The class is built with high casters in mind, not rogues, so it doesn't really translate well to mid-casting types without some modifications. I don't dislike the idea of a mid-casting Rogue, but the execution would be clunky at best without some thought.
I'd grant some of the prestige class features as Rogue talent options or replacements for unwanted class features and make them low-casters who can pick a Sphere Unlock in place of a Skill Unlock, gaining mid-casting with that sphere along with a bonus talent. This would also allow rogues who take the advanced talent for 2 skill unlocks to get 2 Sphere Unlocks instead. With this you keep the Rogue flavor, have the option to pick up Arcane Trickster features, and are not limited to a few spheres but are rather encouraged to specialize. Perhaps allowing them to lower the sneak attack damage to increase the DC of riders on their spells so they can be decent at debuffing if desired.