You seem to have already covered most of the obvious next steps - Razorwings, Grotesques, and Ravagers. Venoms would also be a very good inclusion, so you might want to consider buying some of those as well.
The downside is that these are all very strong units, so they're going to skew your lists away from being casual. Scourges and Mandrakes are pretty good but not quite as cut-throat, so you could also pick up some of those (or Namarti Thralls to convert into Mandrakes).