Necro+Skellies+Dogs need no explanation, I hope.
Grimghasts are what I'm testing as the new hammer. Theory: at 1000pts games there isn't sufficient model count for the opponent to reliably block off all gravesite vectors, so with clever gravesite placement we can go with mKoS in ambush coming in behind/beside the enemy line onto a gravesite - while at the same time pulling up the grimghasts from the grave onto that same gravesite. If cogs have been tuned to +2" charge on that turn, there's a good chance of resolving a 20-man grimghast flank charge into the enemy lines, backed up by the mKoS +1 attack buff (which you don't even need to commit if they fail their charge, saving you that CP).
Of course if you see a horde across the table with sufficient mass to block off gravesite vectors you can play a more traditional game and set up the grimghasts in ambush as well.
The gravesite trick is notable because it extends the effective threat range of the ambush move by a huge margin, as long as you have one in range. This, in turn, is only worth considering because of the combination of factors the Soulwar box provides: 1) a Summonable unit with the right combination of cost-effectiveness and durability, and 2) a Hero that enables and enhances that game plan with a timing-appropriate buff.
Who knows, maybe it's still not enough and the best things to ambush are still Vargs and Harbingers. But at the very least, it's worth trying!