I've always wanted at least 3 new kits. A fully modular Gargant kit with Standard, Armoured and Ranged versions. A Leader kit with the option of King Brodd, Chieftain or Shaman.
The third kit to be Half-Gargants as you mention in place of Armigers but I feel that the Aleguzzlar will actually fulfil this role.
And Scenery, in place of traditional scenery the piece will actually be a Pack Mammoth, loaded with armour and ammo that provides a buff to the Gargants with High Wounds low Saves. Alternatively the Mammoth could be used as a mount for the Cheiftan. There could be large Cave Bears that are used as almost Hunting Dogs.
As for gameplay I would hope that GW learned from their mistakes with the IK.