Ogres would be top of the list for me, good opportunity to resculpt the core units in line with the newer sculpts with bulkier legs, and lots of weapon/ head options on the sprue.
Every army that has a list needs at least one HIPs kit, after that it would be great to see things like cavalry units and monsters get sprues.
I think picking a flagship army or two to almost entirely convert entirely to hard plastic might also be a good idea, looking ahead for 4th edition in the future, to launch it with a brilliant starter that punches above GW in terms of quality/value.
Something like Basileans vs. Undead, adding a dual infantry/cavalry kit for the sisterhood, dual paladin/knights kit and Elohi.
Undead are the easier pick for the other army since they’re the army with the most HIPs kits and the quality is excellent. Could easily add revenant knights and/or werewolves to finish it, or even a dual soul/reaver knights kit to equalise the points values a bit.
Would be possible to have a mega launch box, probably similar in price to AoS : Dominion, but blowing it out of the water in terms of mini count, something to grab people’s attention.
At the moment, Mantic’s sci-fi range has been the one to get fully fleshed out with multiple factions having huge vehicle kits and multiple infantry kits.