This is just from my head and based on nothing. But as, like it or not, AoS in many ways mirrors or follows 40k, I had been thinking that maybe we are seeing the AoS version of the Great Crusade, and maybe it'll initially play out well for Sigmar but all go wrong with betrayal as he nears the completion of his grand plan. Stormcast civil war.
But maybe in AoS case the corruption will be by Death, rather than Chaos, as Stormcast start to be increasingly dissatisfied with the reforging process and some begin to switch to Nagash.
Edit: I should have said, this is in relation to all the posts above about possible future direction of AoS