While 34.11 and previous versions "spawned" sieges had often gob weapon masters, this does not happen in either df2014 or 42.x with the new "marching armies" of those versions and all the goblin sieges through the years i faced in any of my forts are always made of low skilled gob soldiers or recruits.
And considering they take 2/3 years to decide to come to your fortress in 42.01, it's important to note that the current dwarven military training can easily deliver a couple of legendary dwarves in only a couple of years by simply them sparring, transforming those gob sieges that were supposed to be threat a simple walk in the park when a single legendary dwarf can kill a dozen of those invading gobs by himself in no time.
So do you have plans to have gobs training their military skills in their dark sites the same as dwarves does in your fortress to get sieges challenging again ?
I've made note of it.
Armies can still have members with very high skills in 42.04.
In 1053 a marching army that just vistied Savotasob consisted of a lot of low-skilled goblins. some skilled marks-goblins and a single legendary goblin axeman, Sasir Gislaemoth.
My not-so legendary skilled marksdwarf and hammerdwarf squads easily dispatched the low skilled and marks-gobblins plus their trolls.
Sasir ate through all 20 warriors with ease: mostly through head-removing ax chops. Two taverns containing a combined 36 visiting solders then proceeded to get pasted to the fort walls.
Will siege armies ever have different goals?That is, can they ever capture spoil such as taking animals in cages off of piles or in pastures to drag them off the map?
Right now there is only one kind of siege: kill everything that moves. There are already normal fort thieves and several kinds of thieving wildlife. There are a few thieve related goals on the dev page.
Having to keep an army from stealing you blind on top of butchering your citizens seems very medieval. Forts also would have to be very careful about leaving stuff outside the gates. Having a depot destroyed is inconvenient. Having everything that was stuck on it hauled off too (by something other than a random kea or buzzard) means rethinking some fort designs.