I made the orcs as larger, stronger, faster, earlier goblins. They still siege you until you die, but they end up showing up much earlier (first winter or so). I've found that that adds significantly to the game, since you add a new "type" of race: Orcs become the "siege until you die" race and goblins become a "stealth attacking" race thanks to ambushes. Goblin sieges become laughable, since by the time they start actually laying siege, you've been dealing with 80-orc sieges for several years, and goblins are weak compared to orcs. Beyond that, though, I agree that there can't be much variation between sieging races, especially until we can have better control over the training, equipment, and numbers that a siege brings (imagine the difference between a siege of 200 novice wrestlers and a siege of 10 well-equipped Axe Lords)
That said, I think that adding a whole bunch of races to the game isn't very pointful. I could see adding trading races and differentiating them based on trade goods, but that could be difficult to justify/set up, and I'm not sure what the effect of having caravans from multiple races possible in the same season is.
The other thing that I really don't like is the addition of super-races, since those seem to turn into a generic combination of ludicrous size, unreasonable DAMBLOCK, and all the tags that you see on the megabeasts. The reason that I don't like those is that when they show up, you've either got the champion military backed by the Entrance Corridor of Doom to kill them, or you don't. There is almost never a "middle ground" where, through superior positioning or tactics you are able to defeat them (or achieve some other significant objective, like escorting a liaison out). And I think that it's been demonstrated conclusively that skills/stats have enough of an influence that a sufficiently legendary champion can take down anything coming directly from the raws in single combat.