Gridania is the closest to the Garlean Empire's stronghold (Ala'Mhigo) so their biggest threat is probably that. IIRC, the 1.0 story-arc with Gridania felt way more involved in that aspect than the others (until the end when they all unify and such). There is also the Elementals (which were probably the other big bad for the city-state in 1.0 but not as big or bad in 2.0).

Also in 2.0 I don't really get the isolationist vibe anymore (its there at times but not nearly as much as in 1.0) 2.0 really tamed Gridania down quite a bit; however I feel they will ramp back up when we get back into the Garlean stuff as again they are the closest city-state to the Empire-proper.