

Then expand or not accordingly, at best they have the starting point for a cool new army at worst they have done something to make a small portion of their player base very happy. Create a Skaven or Grot sky vessel and see how many people add it to their armies. For example release a hero and unit of Exodites that can be allied to one or both of the Eldar ranges and see how people react.

This has always baffled me, I think that releasing a very small contingent that can be placed into a related army would make it a great way to test the viability of these concepts. Sometimes they are ideas GW never gets around to making (they've been teasing Exodites for what nearly 30 years now?). Skaven and Grots riding Khadoran Airships Eldar riding Seraphon (lizardmen) monsters etc. Don't forget GW will often seed ideas that never get made.
