Coming out in March eh? Well I suppose that is acceptable, even though there are armies that need it more it is a more . . . universal style of army and makes sense that they would be released first.
As for the new units; I would prefer the 'hanger's on' type unit upgrade to be for the regular goblins as night goblins already have a neat little unit upgrade (by which I am alluding to the amusing antics of the fanatics), but I have no say in the development whatsoever so that is just wishing and a small amount of speculation.
Gargantuan spiders sound like a fun Gobliny type of unit.
I do hope the Army books follow in the footsteps of the Rule book in terms of fluff and pictures. I will be interested to see if the common magic items will be reprinted in the army book and if so will any of those items have different point costs to those in the rule book.