The soul grinder now comes with a larger variety of upgrades compared to the previous codex. The first upgrade is that it now must be upgraded to serving one of the four major powers. Khorne will help thanks to furious charge. Tzeentch is not so good: re-rolling saves of 1 is not so hot. Nurgle is excellent for shrouded. And Slaanesh is nice due to rending (but you have an iron claw any way for x2S and AP2).
The other options are similar to the previous edition: phlegm gives a good pie plate attack, whilst warp gaze gives a much needed S10 attack option. That said, we must bear in mind that this is at BS3. And finally: don't bother with warpsword. Really: don't.
It is also worth noting that the harvester has sky fire -- a much needed factor in a pure deamons army. But equally, given allies, I suspect that some heldrakes will be flying alongside daemons anyhow!
My favourite build would therefore be:
Soul Grinder, daemon of Nurgle, Phlegm Bombardment (180 points)
This one has the flexibility of keeping him in the back lines to take advantage of shrouded, or get up close and personal with the ironclaw.