If you're dead set on an Arcane Trickster, go Gnome Illusionist 5/Rogue 1/Assassin 1/Arcane Trickster 2/Loremaster 2/Arcane Trickster 3-6/Archmage 5. That preserves the "trickster" feel while still making you a primary arcane caster with only two lost casting levels.
You have to be evil for Assassin (which is there to get more Sneak Attack without spending 3 Rogue levels), but you can always play this out as "Rather on the mean, 'pragmatic' side, and pretty selfish. Asshole? Yeah. Champion of Evil? Nope".
You actually get a lot of sneak attack, but won't use it much, I expect, except for a sneaky ray spell now and then (augmented with a Quickened True Strike, this might be an option from time to time, even though your BAB is teh suckxorz).
Most of your feats go to qualifying for PrCs, but except for Skill Focus taken twice, those are OK to good feats. Except for a bit of metamagic and item creation, there's not much in core that a caster wants to take, anyway. Maybe ask your DM to treat Skill Focus [Spellcraft], which Archmage requires, as Skill Focus [any knowledge], which Loremaster requires. Loremaster adds the Lore ability, which can be quite nice, and pays back the mediocre skill focus feat via Secret, anyway (if you have Int 24+ by level 10, wich is doable).
Build:
1 - Illusionist (B: Scribe Scroll, Spell Focus: Illusion) {banned schools: Enchantment, Evocation; Raven or Bat familiar}
2 - Illusionist
3 - Illusionist (Improved Initiative)
4 - Illusionist
5 - Illusionist (B: Craft Wondrous Item)
6 - Rogue (Skill Focus: Knowledge [Arcana]) {Sneak Attack +1d6, Trapfinding}
7 - Assassin {Sneak Attack +2d6, useless Poison use and Death Attack, 2 spells yaykthx}
8 - Arcane Trickster {Ranged Legerdemain 1/day}
9 - Arcane Trickster (Quicken Spell) {Sneak Attack +3d6}
10 - Loremaster (B: Spell Focus: whatever, via {Secret})
11 - Loremaster {Lore}
12 - Arcane Trickster (Craft Staff) {Impromptu Sneak Attack}
13 - Arcane Trickster {Sneak Attack +4d6}
14 - Arcane Trickster {Ranged Legerdemain 2/day}
15 - Arcane Trickster (Skill Focus: Spellcraft) {Sneak Attack +5d6}
16 - Archmage {High Arcana: Mastery of Shaping}
17 - Archmage {High Arcana: Arcane Reach}
18 - Archmage (open feat) {High Arcana: Mastery of Elements}
19 - Archmage {High Arcana: Spell-Like Ability [Time Stop]}
20 - Archmage {High Arcana: Spell Power}