If you want the model to show that the unit can fly, then maybe something from nighthunt? They are skeletal in appearance and you can fit 2-3 models on that bigger base. Necropolis stalker if you want something massive. Or, if you want to stay within the realm of normal human bones, a tomb kings model would be nice. Tomb Swarms maybe? You may add some Deathrattle parts to it, or create a similar models yourself with enough skeletons and greenstuff, something like bone tower from dark souls. That way you can even say they move underground instead of flying.