The aranea, first in a long line of 'arachnids with a twist'. In this case, the twist is the aranea's shapechanging abilities and sorcerer casting. Somewhat surprisingly, they aren't evil but true neutral, despite their habit of kidnapping people and asking ransom.
Araneas have bonuses to all stats but strength and innate sorcerer casting, making that class an obvious fit. They have some pretty interesting other abilities too, such as the power to throw webs that completely prevent movement (whereas a regular spider's only entangles the target) or attack with a poisonous bite. Really, everything about this just screams 'GISH!' to me.
So an aranea is equal to a gishy sorcerer of what level? I'm inclined towards a +2 LA (so much for shaking things up), especially considering the Magical Beast HD it gets are far nicer than regular sorcerer HD.
+2 LA for now, unless the hivemind disagrees.