One thing that seems underutilised to me in fantasy kits is a multi-species approach. Given the huge number of 'evil humanoids' in fantasy games, it would be great to get a kit that allowed you to build various types from the same sprues.
An obvious example would be an orc sprue that had both Tolkienesque and pig-faced heads. But it could be taken further, with bestial heads (broos, beastmen, gnolls) and D&Dish hobgoblins. Scantily clad 'tribal' bodies could have optional lizardman heads (and tails!), and so on.
Frostgrave has done a bit of this, with the cultist that can also be assembled as zombies or skeletons. But your typical bulky, humanoid monster body could be a whole range of things depending on the head. If a company were to do a versatile "monstrous humanoids" boxed set, I'm sure there'd be a host of buyers. And those who might want to make Formorians or chaos creatures or whatever could just use all the parts together.